leave the short block alone, ive got a set of good running closed chamber heads 1964 big block 516s 74 cc, your 400 open heads spec out @ 84-88ccs. this would take your 8.5 to 1 400 up in comp to 9.25-10 to1 you can mill 74 cc heads 40 thou with no issues with valvetrain or intake fitment.