I use a thin blade screw driver to rock all the pistons over(stick it between the piston top above the ringlands) so the piston will be forced up against the outside cylinder wall and I measure the inside edge(cam side) of the piston deck and record it, I do the other side with the screw driver and measure the outside edge and write it down. You can average those or use both measurements for figuring the compression ratio on one of the online comp. ratio web sites like United Machine or Wallace Don't be surprised to see between .010 to .004 differences on deck heights depending on the piston to wall clearances
I have found that the crankshaft rod journal indexing, the length of the rods center to center and the piston wrist pin locations all influence the piston deck heights at TDC I've also seen stock Mopar blocks be tapered from front to rear and vary on both sides, Stock production machining tolerances where not set to meet blue print specs
Have you check the size of the combustion chambers, all eight of them, yet? If not do that now if you want to find to how much the compression ratio varies on your motor