Piston 318

Great plan;
I think you can run your Dcr up to about 9/1 with alloy heads. I did/do,(367 cubes) and run 87E10 in that. Depending on your cam, that could be as high as 11.3 Scr, or even more.
You can't get to 11.3 with flat-top pistons down in the holes and 63cc heads.
I think @racerdan80 is on the right track.

But if you get close at least, (to 9.0 Dcr); she'd be a heck of a runner down low.
I've had my 367 to a Dcr of ~9.25 at ~900 ft elevation, which ran gangbusters stoopid strong. and got mega mpgs as well.