There is www.gmtuners.com . I have been working with them for a few months.
I'm not reccomending what i did, but so far:
I'm shifing at 6400<--too high blah blah i don't want to hear it
Tuned close to 100% for L67 injectors and L67 maf
Advanced timing
raised idle
removed evap code
and some other stuff i can't think of right now.
The bad part is that it'* impossible to get all this right done in the first shot esp. when it'* not done in person. But gm tuners won't stop sending me updated chips till i'm satisfied 100%. Plus it'* only like 40 bucks i think.
But a custom chip isn't going to do too much without major mods besides make you run off high octane gas. Maybe like 10 hp after hours and hours and hours of scanning/data collecting and numerous chips.

I'm kinda taking a break from data collecting because of the lower temps. I'm not trying to be tuned for the winter and get all kinds of kr in the warmer summer months.

So with a different rear exhaust manifold, does that mean that only the stock manifolds are different, but aftermarket ones will work? Or does it mean that there is nothing as far as headers go that will work on a '95.

I thought the 4T65-E didn't come in the NA 3800s untill 1998.

The rear manifold/downpipe has a slightly diffrent bolt pattern than the rest of the L36 manifolds. Trust me. I bought a zzp ceramic coated ported rear mani and after i finished bolting it up to the head/crossover, the studs wouldn't line up with the downpipe. not to mention i was freezing outside when i did it. I then had to go and get a custom downpipe witch i got ripped on(looong story). BUT, if you get a mandrel bent 2.5" downpipe or 3" press/cruch bent put on at the same time, there is no problem because u'll be replacing the downpipe flange anyway. The front manifold and crossover should all be the same.