How did you arrive at 10.73:1 compression? Did you actually measure/CC everything yourself? I ask because when I plug in the numbers for a flat top 493 into my calculator with an 84 cc head, .039 1009 4.410 bore gasket -4cc valve reliefs and -.017 deck I get 10.86:1.
The Ross pistons in my 493 were advertised as being .017 in the hole but it only took .007 to actually zero the decks on my '76 block so in actual fact the pistons were only -.007. Now you are at 11.04 if your block was similar. At zero deck you would be at 11.29:1.
I didn't catch all the details on your build so if all this stuff has been already verified and I mean measured by you or your machinist and not assumed because of something wrote on the packaging, I apologize. But if you bought the heads used for instance and assumed they were 84 cc because the PO forgot to tell you he wacked them .060" it would go a long way to explain why your engine is acting like it has a 12:1 CR.
