I am running the same basic numbers and get 9.88:1 SCR. I used 64cc 302 combustion chamber sizes; I have not ever measured them but all the data I can find is that the 302's typically have 64 cc combustion chambers, stock and without being shaved. Anyone have better info on that? Easy enough for the OP to measure the 302's that he has. That combustion chamber actual size is the key parameter to know here. (And the piston eyebrows' cc's...)
I get 8.11 DCR with this cam; the cam has a lot of duration to lower DCR. Sounds doable to me...... and you'll have a decent quench gap to help. (But I have not thought out the tire size and vehicle weight however.)
As for the question on the original pistons and CR: The 'advertised' 10.5:1 SCR for the original engine was not really 10.5:1. More like 9.5:1 from all I can find. It computes to 9.7:1 with a .022" shim steel head gasket and 73 cc open combustion chambers.
