Lots of versions of
4X heads. If the 4X heads are 2.11/1.66 from a 400, go for it. They will be around 98cc. If they are 114cc from a 455, you will be able to pee in the gas tank and still run OK. 4X's
flow about as well as 6X heads.
http://fiedlerh.home.att.net/