Figuring out how much heads have been cut?

Again without pulling the heads to actually see, and measure, what you have, there really isn't any way to really KNOW what you have.
As I said above, Trw made 8 to 1, 9 to 1 and 10 to 1 flattops (the ten to one's had valve reliefs, which yours dont) and domed "11.5 " to one.
It MAY have 9 to 1 trw flattops, and got the 915s for a compression boost, probably why it runs good now (9.75? to one?).
I would bet a bit that it isn't 7.5 to one, LOL.