318?

AbodyJoe has a good point. A Magnum engine or the series just before the Magnum engines came out were roller cam engines, but.....

The '74 will do fine. Once you swap out pistons for some good compresion, you'll be in good shape. The pistons for a 318 are the only expensive part of this build. Since there not as popular as a 360 to build, the slugs will cost a bit more. After that, it's all the same.
Don't worry about the cast crank. The can hold a real good amount of power. No need to get a forged crank.