So if you were to put your 340 J heads on your 318 you'd drop your compression ratio .3 of a point...I’ve heard lots of guys claim more than 6 times that amount!
Those claims didnt think to mention the use of standard .045 thick head gaskets either.lol
stock head gaskets are about .019 thick....so they added about 5cc's to the mix and ended up with a total of 9cc's bigger.