Engine choice

My inclination would be bigblock, if that is your end goal anyway. But it sounded like you have a 318 you could do a cheap valve job on, throw in the 3.23 gears and a decent converter, a set of cheap headers and have fun while building the real motor the right way. I would opt for a stroked 400, and it is about as cheap to build a 512 stroker as any other. With a 512, a hydraulic cam, and even stock valve train if you go with a milder rate of lift cam with enough duration, it will be an animal even with the 3.23 gears. A friend has a 68 Roadrunner, factory weight, runs 11.40s corked up with a 451 , although he was running deep gears and slicks. That was with stock iron heads worked a bit, bowl ported.
But my point is a 3.23 geared striker is a great performing cruiser combo!!