I had almost the same vehicle for a few years - an '88 GMC Sierrra 2500 (3/4 ton). If you want to do anything even semi-serious with respect to making power, get rid of the TBI and go with port injection. I never really did too much serious with the truck though - didn't really see the point. Even if you make 400+ h.p., it's still in a vehicle with terrible handling characteristics and that weighs twice as much as a sports car with the same power. Not my cup of tea.
There's a good forum here though:
http://67-72chevytrucks.com/vboard/