264 and 322 heads are the same but there is a difference between early and late '55 heads; early '55 heads were the same as '54, late '55 heads were the same as '56. It is my understanding that the piston dome was changed when the heads were changed, but I don't know any more detail then that. Perhaps Tom Telesco (telriv) would know. I do believe that the heads will interchange, but there might be an advantage to using the early heads or the late heads - depending on whether you have an early or late engine.
The pair of heads that I have is casting # 1168930, which looks to be a '56 casting. Let me know if this works for you.
