340 compression ratio

most find stock early hi comp 340's were around 9.5:1 with 72 cc heads and pistons out of the deck 20 thou or so. Mopar rated them at 10.5:1 so you could blueprint them to 10.5 and be NHRA legal.( cut the decks, cc the heads down to 63-65 cc's etc.)
If your pistons are at zero deck you would be 9 - lo 9's and pistons below the decks like later 340's will give you something in the 8's.
But the only way to be exact would be to measure and calculate.