+1 for Black Bear Performance. They've been very helpful with the tune on my '97 C1500, including making the necessary changes to swap from a 4L60E to a 4L80E transmission. They've altered the tune in that truck over the years and never charged me anything for it, I just paid shipping on the PCM.
On that note, I would strongly recommend just paying their core charge and effectively buying a spare PCM. That way if you ever need/want to make changes to the tuned PCM you can just pop your stock one back in while you wait. No down time.
Black Bear may be reluctant to alter emissions equipment behavior these days, it seems like many mail order tuning businesses are. Years ago they had no problem with it but I haven't asked recently.
If you're looking for tuning software that would enable you to make changes yourself, I believe JET DST is the only real option for the '96-'97 PCMs.