Milling Pistons ?

that is what you have to do....1/2 inch fill on the cylinder so you can cal the dome volume...

but as said above...a 68-71 340 piston is usually out of the deck anywhere from .008 to .020 ...and adjust the head gasket accordingly and you will have a 10 to 1 compression..

I got a set that had the dome milled mostly off...but they are in a old 340 in the corner of shed...you can just make out the dimple for the spark plugs still...