OP, you may want to look at NHRA web site for the 1968 to 1971 340 piston deck hieghts and cylinder head combuston chamber sizes and use those for figuring the original true compression ratios. I built several 1971 340 NHRA legal "stocker" motors for a customer back in the mid 1990s, using NHRA specs and doing the math those motors had a true 11,5 to 1 compression ratio in bluepring conditions I don't remember the exact specs. now on how high the deck of the pistons where above the cylinder deck, the head gasket thickness came into play so you could raise the pistons up with a thicker head gasket or lower it with a thinner gaskest On your deal your going to have to have a custom set of pistons made to get the exact compression ratio you want. Which is common and normal when building destroke or increase stroked motors, regardless of brand and size IHTHs
BTW, Are you aware that when yuo destroke a short stroke motor to start with you will loose all the bottom end power that it had? If not maybe increasing the stroke for your deal might be better for a pump gas street and srip car The lack of bottom end power was probally one of the main reasons Mopar came out with the 360 C.I. motors in 1971 for a lot of thier cars and trucks, more C.I., more power and torque