As far as carbs go, a Holley spreadbore will probably be perfect for you. Models 4165 and 4175 were designed as direct replacements for the Rochesters that most GMs came with. They came in either 650 or 800 cfm, the 650 should work good for you. It oughtta be a good compromise between economy and power. The spreadbore is nice because the small primaries use less fuel when you're just cruising under light throttle, and with that 355 in that small truck you're not gonna need a whole lotta throttle most of the time. You can find these carbs on Ebay or at swap meets usually, but plan on a rebuild where ever you get one from.
I was wondering, about swapping a V8 into our little trucks, would the weight of the motor make the front end sag a whole lot? I'm guessing since V8 would weigh almost twice as much as a four cylinder, you'd have to swap in leaf springs from a V8 truck. Is that right, or would the standard 4-5" lift Toy springs handle the weight?