Those are probably an open chamber head since they came on truck blocks and tall deck blocks. www.Mortec.com has been really accurate for me so far. The only way you're going to get decent compression with a 396 is with a closed chamber head. The small bore size is what kills it with an open chamber. You can run a closed chamber dome with an open chamber head but not vise versa. The biggest dome TRW makes for the 396 I think is a 35cc closed chamber dome. Not sure what that would give you for CR. The other option which I had already looked into is that JE will make you any piston you want but it will cost $900+ bucks or so.