the 360 is .040 over already, cylinder walls aren't perfect although it doesn't burn oil. It only has "8:1" pistons with the magnum heads, and well the bearings are really worn too...been running 14psi hot oil pressure and not climbing quickly when revved for the last two years.

I did get a good deal on this 340...forged crank, good cylinder walls, etc...heads/intake/block/& crank are worth more than I paid for it anyway. Not a risk. I'd like to run it more than the 360 as its going to be higher compression and much more servicable.

The pistons are down the hole in that 360 (they are flat) about .1"

This 340 with them being .005 down the hole (they are supposed to be .007 above based on the quoted compression height) is worth about 1cc compared to zero deck. and J-heads have been between 64 and 68cc depending on who worked on them. Really it depends on the cc of the valve reliefs.

with 10cc worth of valve reliefs:
9.97:1
with 15cc worth of valve reliefs:
9.43:1
with 20cc worth of valve reliefs:
8.95:1

I used the 4.070 bore for all the calcs other than the head gasket which is 4.1", 68cc chambers, .039 compressed gasket thickness, and .005 below deck.

Anyway, my point is that the 340 with the magnums or even with the "J" heads will have more compression than my current 360.