The NHRA blueprint specs. on the "stock" 1968 to 1971 340 called for between .015 to .030 positive deck height depending on which head gasket you used, that calculated out to be 11.25 to 1 compression with 63.?(CRS now) head CC work
We had Ross make us some NHRA acceptable replacement legal pistons and then back cut the rings and so on work

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)