early 273 pistons.

That makes a difference. I ran the SCR with 58 cc chamber volume, -.020" deck height and a .021" think shim head gasket with 3.8" gasket bore, and came up with 8.75:1.

But I cannot see how the '68 2 BBL open chamber would be 9:1.... I cc'd 2 holes on one head off this '68 2 BBL 273 and they were 68 cc's. (675 heads). With the .021" shim gasket, .00" deck height, and 68 cc chambers, that works out to 8.1:1. I don't think your info on the later 2BBL 273 is correct.

BTW, I assumed 7 cc in eyebrow volume for these latest calcs. I've never actually measured them.