The 305 is not worth putting the money into because it is too weak of a motor to build up for power, put your money into the 350 for the same cost of the 305. If all you have is the 305 just build it to stock and live with it. A stock 305 is 140hp and a stock 350 is 160hp for mid 70s and up. Early years had upwords of 350hp for the 350. GM derated the engines back then and started using cheeper metals.
Get yourself a wrecked 1990 and later truck, salvage the engine. The 1987 to 1992 camaros and firebirds had roller cam engines rated at 180 to 200 horsepower stock. Tuned port engines look nice in older cars and trucks.
Well my 305 is from a 91 camaro. It was rated at 230 hp stock but it has an aftermarket cam and the tpi was ditched for a carb so I'm not quite sure how that would effect it. When its running right, it has enough power for me. More then anything I only put it in because I felt strange having a SS with a v6. I needed sound to match the looks. I plan on doing a swap thats a little more worth while later. Either a 350 crate, 454 vortec motor, maybe even a 572 big block? Don't know. My plan was to put the 305 in and hope that it lasts me my senior year of high school then buy another car for college and give the camino the engine and body work it deserves. So no, I wouldn't go building a 305 for performance, fuel mileage, or anything special. It really is a "middle of the road" engine. Trustworthy though, and built like a damn tank!