An affordable option to the $2-3k FAST systems is Fitech at $1k. Anything over that 350hp, I'd recommend switching to an Edelbrock replacement TBI or a modern TBI, like the FAST system. If you get more then 350hp out of it I'll be surprised (from the stock bottom end that is), but if you do the factory TBI won't support it. Now if you're just looking to breath some new life into a tired old truck, a LT1 hot cam, headers, ECM tune (few places still do it for TBI's), under or over drive pulleys and some cheap mods to the TBI are a good start (Look up Ultimate TBI mods for some good info). Don't get me wrong 350's have potential, but dollar for dollar the LS will save you in the long run. If it's a lot of power the best power adder to an old 350 is to pull it and drop in an LS engine.
Are you looking for a lot of power, or just to wake the truck up a bit?