The 68-69 pistons in the 383 Mag were .020 above deck, which, when combined with the 906 heads gave
a cr figure very close to 10.00 to 1. The 67' and earlier 383's, 2 or 4 bbl, were right around 9.0 to 1 range with 516 heads. One problem is that you cannot put the 516 head on the 68-69 383
Magnum, (ONLY with factory pistons) WITHOUT checking deck height FIRST (closed chamber head vs open chamber head) since there may be interference with the higher compression height on those year only factory pistons. Otherwise, 516's aren't a bad way to make some power, you just can't make the bigger numbers like with the 906's and later heads WITHOUT doing some extensive rework to them.