i begged to differ oldkimmer those 516 heads also came on 413 in 64, as far as rest yes there a closed chamber head with no flow, the thought after 915's were your 383/440 magnum heads!!!! 516 were a dime a dozen, if your going to stay with iron heads step up i'd go to a 906/452's to bring your compression down to like a 9.8/9.5 ratio so you can actually drive it, 516 closed chamber and 915's were on your 10.25/ 10.50 compression motors when you could get gas to run'em back in the 60's/70's with no pinging!!!