marine pistons should be fine, but ditch the 20 over head gasket, unless you decked the block or the head alot. its killing your compression. not sure how much the marine pistons lower it, mainly they have a larger bowl for the wider spray of marine injectors or higher timing. anything over 18-20 degrees of timing is going to kill your bottom end for a stickshift.
88 chevy crewcab 6bt nv4500
72 scout 60/14b 40's