late 318 pistons have a depression in the center, which is called a dish. at .022 deep that may be 3cc's
adding it all up; 13+3+8.6gasket+63heads=87.6cc and at stock 3.91 bore this maths out to 8.45SCr......... not too bad, in theory.
Even if the dish is 6cc, you are still at 8.2
