I don't think there is a specific header to use for the Apollo. I have herd they ARE a pain to install.
Hooker's 1997 catalog shows a Super Competition header for the X bodied 73 - 74 Apollo but it appears to be for the Chevy motor. They do/did list a Buick one for small and big block for the 68 - 72 Skylark (A body) models.