Personally I would be attracted to the 331 Caddy engine and whatever vintage speed equipment you could lay your hands on.

Unfortunatly with any vintage engine (with the exceptions of the SBCs of course) NONE of them are cheap to build. The parts appear to be out there to do them but by the time you get done I believe that you will be looking close to the same money no matter which way you go.

I don't know how much pricing you have done so far but just some examples from E Bay for the engine master kits:

303 Olds $1247.43

331 Caddy $1476.14

331 Hemi $1194.37

You machine work and trans adaptors will also likely be close in price.

The only advantage I can really see here would go to the HEMI believe it or not. Primarly because currently used speed equipment is so readily available for these engines.

You could cut the costs somewhat by building the engine over time like I did and shopping for bargins......but do you want to wait for 15 years to hear it run like I did