Snowman, get the 400 Chevy. However, like I stated in a previous post.... rebuild the whole thing. Don't cut corners just 'cause "some guy" says it runs fine, just a little smoke on start-up. You'll kick yourself in the A$$ if she grenades the first time you wind it up.
Here's my personal experience. I got my donor engine from a VERY good friend of mine's old farm truck. A '76 GMC 3/4 ton, 350 4-bolt, steel crank with 80,000 original miles. engine ran great, no smoke on start-up, idled smooth, no misses, nothing. Well, I considered your idea too. New cam,intake,headers & carb, & leave the bottom end & heads alone. Yea right!!!! As soon as I tore the intake off, all I could see was 30 years of sludge covering everything. I then decided to take the heads off & discovered the cylinders were ridged so bad that the .030 over-bore just cleaned them up, almost had to go .040 OR .060 Another thing to remember is these 70's era engines have a PATHETIC comp. ratio of 8-8.5:1 if you're lucky, & big combustion chamber heads (76cc) Which is fine for turbo or supercharging, but without you better get some flat tops or domed to get the CR up to at least 9.5:1
Sorry for the long rambling, but I just don't want to see a fellow rodder F-himself by trying to save a few dollars. :wink: