I'm going to add in my experience. The write-ups are great, follow them and you'll be OK.
I just got done changing my stock ones out for Prothanes. It took me about 15 hours, but there was some time wasted since I don't have a garage and I have to move all of my tools/parts inside when I take a break so they don't get stolen.
The toughest part for me was re-aligning the engine so I could put the long bolts through after I had the clamshells bolted to the engine. I ended up not using the jack, and just having a buddy pry between the frame bracket and the clamshell. There's that little double-hump in there that works great to keep the pry-bar in place. Pry like heck, get the bolt on one side halfway in, then pry the other side and slide the bolt all of the way in. Go back to the other side and whack the bolt with a hammer until the threads catch, then just crank it through.
The install was painful, but definitely worth it. Shifter movement is very minimal now. No noticeable increase in noise. Y-pipe banging on the floor is totally gone. And it even reduced my drivetrain lash a little bit.
how much easier would takin off the headers make it? i gotta do this in the spring when i do all my other 100k maintence stuff
Sorry, I know this is a little late, but the answer is ALOT easier, disconnect them at least. I would have just taken my headers off if the Y pipe wasn't so stubborn.
Well, i got mine in...after about three days . Maybe 18 hours of work all together. Serious fun! And by fun i mean the easiest thing i've ever done. And by easiest i mean the most difficult thing i've ever done to a car by myself.
Some notes: Did it with the headers and starter still on the car. Ground one of the bolts down to a fine point (very nice trick...thanks to whoever first said that). Had to install the drivers side bolt in backwards...for what ever reason the back hole of the pedestal lined up easier than the front. Prybars are awesome. 3/8 universals are worth a hundred times their weight in gold. And I never want to do that again.
To all who are woried about poly mounts destroying your DD friendliness: Stop worrying and just do it. Its my only car, and now it feels crisp like a BMW...a bmw that makes crashing sounds when the door closes... But seriously, the shifts are more firm and the car doesnt feel mushy anymore. I dont feel any vibrations either. Maybe i will when the poly trans mount and torque arm bushing go in. If it is making more vibrations i just cant hear it because i have this thing that makes a lot of noise already called an LS1.
I couldn't wait for the prothane mounts so I did my H/C swap. at the same time I installed headers & y-pipe. I did it in my garage by myself. I installed headers from under car and had to remove long bolt from motor mount to lift engine up high enough to get one header in position it bolt up. other side mount was still bolted, so one side can be done at a time.
one hell of a good write up. I think "LS1TECH" should give a free shirt or something when ever some one writes up a how-to and gets so much response as this one did. .02
Santa just brought me solid mounts. Do I need to change the trans mount to poly? currently I have the 1LE stiffer mount on it.
You don't have to do a trans mount. You can't have a poly trans mount with stock motor mounts but you can have poly motor mounts with a stock trans mount.
Basically you don't want your motor moving more than your trans.
Trans mounts are really only necessary if you're shifter is all over the place or you have an aftermarket TC. A poly trans mount will probably contribute more to vibrations that the motor mounts, but not by much.
Just got done with mine and wanted to add a hopefully helpful "tip". When putting the clamshell back together, make sure that the holes line up PERFECTLY. Mine were off just a hair which had me shifting the motor and trans all over trying to get things lined up. After a couple hours of struggling, took it apart and realized that the holes weren't perfect, causing the bolts to go in on a slight angle.
Took the clamshell apart and got things lined up. Took 15 minutes to get it in after that (with long tube headers even)
my craptacular TSP catted Y. Trans cross member. BTW, i have a bumpstop too. This Y pipe has two settings:
1) Ground Clearance and bump/vibrate
2)Cat cruncher and no bumping.
Needless to say i am going with a custom 2.5" true dual over or under the axle.
Dont listen to me though, i think i just have extremely bad exhaust luck.
A few days ago my borlamouth fell off on the interstate. When i opened the door to go fix the prob on the side of I81 the wind from a semi ripped the door out of my hand and sprung the door. Yeah, unless that thing gets fixed soon ill bust the paint off the door and fender where they meet now.
I am about 90% completed with this poly motor mount swap. We got all the way through it...driver's side is all bolted up and got the pass side bolted to the block. Now trying to get the 18mm bolt lined up...the hole is 3/8" off.
I read through this writeup and will now be loosening the drivers side 18mm bolt and backing it out half-way...maybe that will belp us get the other bolt started.
I worry about hammering on the bolt...won't that damage the threads? I'd be so f-ing pissed if it did and I had to go get another bolt and start over.
Some of my woes/thoughts:
I also had issues with the two halves of the clamshell not lining up perfectly...had to remove the passenger side mount again and line things up in order to get the bolts in the block.
The passenger side is 11ty billion times harder than the drivers sides....zero room even with the AC and starter out of the way. The drivers side took about 3 hours....spent another 5 on the passenger side and still haven't finished.
Taking the tranny mount loose does not help get things aligned AT ALL!
You will need a huge prybar...get one at least 18 feet long and find an 800-lb gorilla to operate it.
On a 99 Z-28, there are 4 bolts on the AC compressor...I also removed the compressor bracket, which is another 4 bolts.
On a 99 Z-28, there are 4 bolts on the alternator...not 3.
Be sure to take the electrical connections loose on the alternator and the AC compressor....the wires are tiny and they will snap.
Working on your back with the car on jackstands for 8 hours blows...Get someone to help...I had a friend helping me out (thanks Robert P.) and I can't imagine doing this job by yourself.
Good Luck!
Update: I finally finished...we loosened the drivers side and backed the thru bolt out halfway, then used two prybars to get the passenger side bolt through. Then we used a large prybar to get the drivers side the rest of way through. Took 15 minutes with the right parts!
I'm so glad this is over with.
JC
why cant you have just a poly tranny mount with stock motor mounts? seems like it would stiffen the whole thing up and maybe help the stock motor mounts last a bit longer? plus it would be much easier and i'm lazy. my car has stock exhaust but i'm afraid i'll wreck the stock motor mounts on the nitrous and thought this could be a cheap/easier way to help this a little.
why cant you have just a poly tranny mount with stock motor mounts? seems like it would stiffen the whole thing up and maybe help the stock motor mounts last a bit longer? plus it would be much easier and i'm lazy. my car has stock exhaust but i'm afraid i'll wreck the stock motor mounts on the nitrous and thought this could be a cheap/easier way to help this a little.
could someone explain that abit?
I ran a Prothane trans mount with stock motor mounts for about 3 months. I did notice a better shift feel with no ill affects. I had Spohn solids waiting to be installed though. The solid motor mount/Prothane trans mount combo is awesome.
. Maybe 18 hours of work all together. Serious fun! And by fun i mean the easiest thing i've ever done. And by easiest i mean the most difficult thing i've ever done to a car by myself.
I could do this every weekend... 
