[25stevem]

It’s my understanding that those pistons do not come with a chamfer from what I just saw on the summit site, so that chamfer must have been added.

I would bet that the chamfer was done to add a volume that would allow a head of less then 90 CCs to be run on a 455.

Unfortunately to nail down a compression ratio with any head you might choose to use your going to need to CC one cylinder with the piston close to TDC.

A makeshift way to do it would be to get. A 100 CC short type ( wide ) plastic beaker , get some clay and then fill in and level off with a razor blade both the chamfer and the valve notches.
Removing and balling up the clay and dropping it the beaker filled up to the 50 Cc level would then by means of displacement tell you the total CCs you are dealing with to use in a compression formula.

By the way, you do not need a rotating assy, just new pistons and a balance job, and yes that’s about a 700 buck cost assuming you can reuse the rings.

Another possibility is that running the -4 heads now might get you a 8.5 comp and since your goal for next year is to just use the car then just put that RA4 came on the shelf for latter use in your next motor and just get a smaller cam for now.

Something close to a 068 factory cam duration wise but with more lift should work with a 8.5 comp and still give you some tire burning fun!