The factory overstated the compression ratio, a 66 with closed chamber heads probably has a piston that is .080 below the deck, you do NOT want to cut the block that much.

I'm unsure what they did in 66 , but for 68-69 they had the same stated compression ratios for 2bbl and 4 bbl and the pistons had the same compression height between these two engines, the difference was the 2bbl piston had a dish to lower the compression. Your engine probably has closer to 8.0 compression, using 452 will make it a lot lower, it would be cheaper and better to just put hard exh seats in the 516's and install 1.74 exh valves at the same time .