I've done the 4L60E to a TH350 conversion on my buddies 94 T/A and it was just a weekend toy. With out being built the TH350 isn't much stronger at all i beleive and it took a crap in a couple months one a all motor car with a LT4 hot cam and now i just did a T56 swap with a good clutch. I would say find a good builder and just build the 4L60E or do the 6 speed swap. Just my 2 cents.
__________________ 94 Z28 T-top Leather- rebuilt, cammed, and stuff bolted here and there..
If you look at doing the swap your self it's not bad at all, you would need to look around the for sale section and try to find some deals and sell your auto setup to get some of the money back.
__________________ 94 Z28 T-top Leather- rebuilt, cammed, and stuff bolted here and there..
the better out of the two a3s is the th 400. i just picked up a hella built one for around 1k used. my dad has a stock th 400 in his 72 long bed with a big block, 3800 converter and stickies and we have been thrashing on it for about two years now! a t56 cost around 1k a good clutch is around 500, then console, pedals, shifter, misc stuff.
_____I guess it depends on the tranny builder. Any of them can take a dump. I bought a built 4L60E from a reputable builder and it took a dump after less than 5K. The tranny never saw a race ever. (This was a '94 I had for awhile) Had a local tranny builder take it apart and he found it was assembled all wrong. He called the tranny builder to let them know and they said, "Oh, the guy that put that one together is gone. We fired him. Take care, <click>". They hung up the phone. No offer to help defray the cost, no warranty offer. Nothing. Oh well.
____On the other hand, I put a $650 TH350 that had been rebuilt with a 'fake' manual valve body. It lasted thru 20 plus bottles of Nitrous, several pairs of Nitto DRs, one pair of M/T ET streets and One pair of Hoosier 28" slicks. it was on the car from 2000 till late 2003. I went 11.1 @ 121 mph with it (1.5x was the best 60 ft) Only reason it went out is I let it get low on fluid. I now have another, built for 800HP by Century Transmissions in Richmond, TX. The reason I went with the TH350 is;
Strong (Good friend of mine with '70 454 Nova w/Nitrous did wheelies with his TH350)
inexpensive
Lighter than a TH400
I can use my stock length driveshaft (if you use the 9" tailshaft version of the TH350)
th350 or 400??? help 
