Help me diagnose why my head gasket blew

Damn. It's becoming a chore to find trustworthy machine operators.

I feel like Edelbrock needs to ramp up their quality control. I almost bought an air gap manifold from a swap meet for dirt cheap. Measured the runners, side to side to find about 5/32" more distance core shift in the runners on the right side. Over 1/8" and I'm not even sure you would get a runner divider to seal on the shifted runners.

Anyway, I think you made the right move with the new block/ stock deck.

Those pistons should run fine on a square zero deck or even in the hole a bit, on that gas. I've got a 9.75 @ .012" below deck on a +5cc piston, so I do believe that 10:1 is right at .015" on these engines with a flat piston.

It looks like the quench on your pistons was designed for zero deck, check the dish cc on those and punch some numbers into a calculator, with the gaskets you want, before you have everything cut.

Also, for what it's worth, I know that bad quality fuel is an issue here in CO., A friend of mine had a lot of lab tests run on water content from a lot of stations here and he and his master technician from his school discovered that the 87 (mid grade) has a much more stable octane rating within it's numbers and a lot less likelihood to have water in it, because of it's extra refining. It's not much higher than 85, but the quality wasn't even comparable from his lab results.

He also said that gas from small stations with older tanks were significantly cheaper in quality with higher water content, on 85 octane.

Also, octane boosters are a waste of money. They claim to raise octane rating a point or two or five or whatever. The truth is, they do. They raise 87 octane (as an example) from 87 to 87.2/ 87.5, etc. The labeling is very misguiding.