New to the mopar world,Need help with 318 build..pleeasse..

To check how far the piston sits below the deck you need a dial gauge.
Magnums have roller cams but you can put in a flat tappet cam with the right push rods just long as your cam doesn't lift much more than .500" with 1.6 rockers or you need to get the heads machined for higher lift and you can always switch back to roller later if you want.