Cheapest way to built a 300-350hp small block

Low-ish mileage junkyard 5.9 Magnum, magnum specific Airgap intake, and your carb and headers. That should make 300 hp, and definitely on the cheap. Then, if you want, throw a small cam in the 220 @ .050" range with .500"-ish lift, and there's your 350 hp. Easy peasy.