you can't really use 360 pistons in a 340 unless you use a 360 crank and make a stroker which is what you are doing
so you need to run 360 +.060 pistons 4.060 or 0.20 over 340 bore so forget it's a 340- it's now a big bore 360 as far as pistons go
with the x heads use the kb quench dome pistons
with the closed chamber heads you need to run a dished piston to give you a true 9 to 9.5 to one with that cam
that's with running a 0 or close to 0 deck clearance with either piston to give you some quench
if the piston is say .010 down then use the Mr gsket .028 gasket
if 0 then you can use the thinner fel pro (not the 050 thick one)
there are ballance issues with either combination
I've been to Sweeden in December- i would not run an air gap
so what compression height piston for 0.0 deck you need to know this
then how much dish to give you the compression you want for the rev range of that cam
what exhaust?
who is CCA on the rocker arms?
