Okay, so assuming it is a stock engine, with stock pistons, at stock specified deck height, at stock 8.4 compression ratio, how much of a difference would cutting the head 0.060" make ? That's an 0.080" cut with a 0.020" thicker head gasket.
Where did you come up with the 8.4 CR number? The chrysler factory manuals list 8.2, and 8.4 (depending where you look). The question is how did they come up with different CR numbers, when the engine specs (that affect CR) never changed.
Just pull the head and measure what you have. Then use the formular to determine how much to machine.
PS: A slant six head needs to be milled .0066 per 1cc change