**Milling .030 off '920 - 273 heads**

Is the 8.8:1 a printed spec? If so you need to measure how far below the deck the piston actually is and calculate the CCs.
It's best to know the block CC first to calculate what head CC chamber you will need to reach the desired comp ratio. If street car stick with 9 to 1 for pump gas.
Once you know the CC required for the head have the machinist mill the head to meet your spec.
The other thing you need to look at is Piston to Valve clearance when shaving heads and running a thin headgasket. Most of the early SB motors had flat tops and no valve reliefs. .040 milled, .025 or 30 less headgasket thickness will place the valves closer to the piston.
Build the motor to run 8.2 Dynamic Compression and it will make real power and run on pump gas.