the bores are both 81mm
the only thing that you might have to do is machine the rods to accept the pistons...Im not sure on that one
the compression is roughly higher 10:1 compression in an ls w/ usdm pistons
11:1 w/ jdm p30's
(see this site for calculating compression -
http://www.c-speedracing.com/howto/c...c/compcalc.php )