72 455 Ga Heads

I have a 72 model 455 with Ga heads. The problem is, the casting number is 408100 and I can't find any information on this casting. I found the casting number 409100, but no where can I find the 408. Does anyone have any idea's? Maybe because the exhaust ports are the 1.685 rather than the 1.625, I have no idea.

-bob