www.wait4meperformance.com
I order a tuned PCM from him for my 2002 GMC 1500HD 6.0L, WOW what a difference! I am going to get one for my Dad's 98 GMC with a 4.3L as well.
If you get a tuned PCM (core) from him it is $159. If you send him yours to be tuned, it is something like $75.
His web site isn't much, but just give him a call. He is a good guy to deal with, and will answer any and all questions you have.