Do you have 2.7cc valve relieves or 12cc?

Plus there is a lot of "I think" in there, you need to really measure anything that has not been. A stright endge and a feeler gauge at TDC is better than nothing, and recommend you do at least the 4 corner holes because rarely is a deck that hasnt been squared decked even close.

However, with 12 cc I get 8.6 too, with 2.7 I get 9.71 SCR, even thats a smidge low

However, DCR isnt the only number, as you rev higher the motor gets closer to 100% VE, and even exceeds it. That means it will be filling the cylinder well above the DCR number and even exceed the SCR

If it were be, I'd go with the 2.7's, be sure to degree the cam and have it installed no later than 106 ICL, you could even run it at 104 if you have P/V clearance, and maybe find a .020-.022 head gasket

However, dont guess at the cam installation, the deck clearance, the valve clearance, , the cc'd of the heads, or if the deck if straight. Once we have the info, even if its ugly, we can deal with it. If you dont check it, all the math and money is a waste

There also may be another option, if you order a custom grind from Comp you may be able to find a cam with steeper ramps (less adv, more .050 duration) if you do that, you will need less SCR for the same DCR

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70 Sportsroof, 427 FE/489 cid, 10.7:1,Ported Edelbrock heads, and RPM intake, 1000 Holley HP, Ceramic Hooker Supercomps, TKO-600, 3.70, A/C