I have a 07 sierra 1500 on 8psi(procharged) and second gear went out and first is probably not far behind. What stock trans. is in it now and any suggestion on what I should be in the market for? I"ll probably be around 500-600hp when the truck is done.

 

Your stock trans is a 4L60e, you could either drop in a built 4L60e and worry about it someday in the future again, or swap to a nearly stock 4L80e and forget about it. I'd look into a 4L80e with a Transgo shift kit, it's what I'm doing on my 2007.