Cam for this 340 ?

YEP! Me too. Also, since you allready have mildly done "X" heads, I'd look into further work on them rather than go aluminum.

If you have not yet gotten pistons, I would look at a max of 10-1 with adecently sized cam to bleed some pressure off. A good example would be the KB107's. with a "X" head chamber at approx. 72 -74 cc's, your qatio will be 9.8-1 (W/ a 74cc chamber) This is with a Fel-Pro .039 thick gasket. You can get a Fel-Pro gasket at .050/.055 thick as well to take the edge off if needed.