340 Direct Connection cylinder heads

Does anyone have a part number for the Mopar 340 cylinder head from the 1970's that was cast iron but with no heat crossover passage? And another question, how much (if any) compression would be lost putting these heads on a 273 with 10:5 compression factory style pistons? Thank you.
The reason I am researching this is that back in the late 1970's, I tried this already once and my ET's went from high 12's to low 13's and I was really befuddled, not to mention disappointed!