Figuring out how much heads have been cut?

I picked up a new Performer RPM intake and when I tried to install it, the bolts holes were all off 1/2 a hole in height. The M1 that came with the engine ended up going back on, which was also a challenge to get the bolts started, but eventually they all fit.

I am assuming since the new intake didn't fit, that either my heads were milled or the block was decked at some point over the years and the mating surface on the heads weren't adjusted to match.

Is there a good (easy) way to grab some measurements with the engine assembled to determine how much metal has been removed? I thought I read somewhere that the outer head bolt flange surface was 1" thick and that can be measured to see how much was milled off.

Is that correct and is there a similar measurement to take on the block?

It's a '69 440 block with 915 heads.