There's a guy that frequents (or at least use to) another Pontiac site, who is considered an expert in transmissions. He owns http://www.jakesperformance.com/ and has building them for years. He only recommends the TransGo kits.
He follows multiple forums (all GM makes), so google "jakeshoe" and TH350 and you'll get a ton of hits. He has lots of great advice that he's shared with members that had questions, with some interesting tid bits and recommendation when using those kits. So read up first before you buy and install.
I have done a few, and they are fairly easy to do. I think that the TransGo kits are the best, and easiest to do. It has been awhile since I have done one but I think I remember that there are a couple of shift firmness options you can do, I have always done the most firm shifts on mine