My requirements... what to build?

You could stroke the 318 and go with a GM 200R4 auto. Nice over drive and you don't have to cut the car to pieces or buy expensive transmission control. That would leave you with the "numbers matching" engine......you did mention that as if it might be a little important.
Do you need a mounting bracket for this?
i thought i read where you mentioned this in another thread and there was something else that needed to be fabricated.

OD is not out of the realm yet. Just making the hard choice on the 318 or the 360.

I really need to talk to a builder LOL