As much as I'd be interested to see what a 390 would do, I gotta throw my vote to the stock stroke 318. It would have a lot more relevance to 99% of the guys bolting one together.
Ever consider a 273 furthehelluvit? I'd love to see one on the dyno wound up good and tight...
The piston cost for a 273 kills that idea. Its either the egge cast ones with a terrible ring package or custom. And I would have to sell the engine when I was done, and I believe a high strung expensive 273 would likely never sell