how does messing with the timing change your fuel pressure?
fwiw i would first start with the fundamentals, can you check your fuel pressure at w.o.t. ? If the engine isn't getting fuel that will cause your stumble and lack of top end performance for sure. How are your spark plugs, are they a tan color? If they're black and sooty then they are dry fouled from being too cold a heat range. Although expensive, i swear by iridium spark plugs, they fire easier than conventional electrode plugs and would help at high rpm if that's where the problem is. What you can also do first is simply narrow the gap on your plugs 0.005" if they're currently gapped > 0.045".
for the adjustable timing light, what you first do is set the dial to whatever base timing you want. Let's use 10 degree BTDC, so set the dial on 10. With engine at idle, no vacuum advance hooked up, you should read 0 on the timing mark on the crank pulley. As you turn the timing light dial back to 0, the timing illumination on the crank pulley will move towards 10 dBTDC. With base timing set rev engine up to 3000 rpm and illuminate the timing mark on the crank pulley, turn the timing light dial upwards past 20 until you being to see the timing marks illuminated on the pulley. Adjust the dial to illuminate 0 on the pulley, then whatever the dial is at on the timing light is your timing.
i kinda disagree with the exhaust being the problem, a 3" is not too small. Also, if the exhaust is choking the engine then that manifests itself as the engine hitting a brick wall at a certain rpm & load, not a stumble. If the other stuff checks out, then i would look into valve train next, specifically the springs and your lifter preload and seat pressures like was said. At that rpm, springs and valve train as a whole plays a very big part. You mentioned you used the springs that came with the head, the springs should also be matched with the cam & rocker arms.
http://www.chevyhiperformance.com/te...ide/index.html http://www.compcams.com/technical/FA...lveSprings.asp