Those 915 heads will help boost the squeeze. The 1977 400 and 440s were rated at 8.2 for standard and 7.8 for police. I have no explaination why the cop cars got less squeeze. The 915 heads will likely raise the compression ratio by 1.0 to 1.5 points. A mid 9 to one ratio isn't bad. You will make decent power and still be able to squeak by with regular fuel in cool weather.
If I ever have to replace the pistons in my 440/493, I'm lowering my compression from 10.8 down to the mid 9s. It will make less power, but I'll also have a little flexibility on the road if "Joe Blow" gas station only has dogwater 87 octane.
