Make sure your pistons are below deck. With the closed chambers on 915 heads they may hit the heads. Early HIPO 383 pistons had a positive deck height.
INCORRECT , there was not a 383 built with positive deck height, NHRA spec gives a positive deck height and NHRA spec doesn't come close to actual factory specs ... except for cam lift...
The pistons look like KB 162's , they should be .024 below the deck in the block has not been decked a ton . Ring end gap on the top ring is critical with those pistons .