[70.5_firebird]

I just bought a 1975 pontiac 400, but it has mismatched heads on it.
The block has a build date of A275 - Jan 27, 75.
1 head is a 6x, build date B245 - Feb 24, 75.
1 head is a 5c, build date J304 - Oct 30, 74.
From what I can see online, both heads are 2.11/1.66, 7.6cr and 98 cc's.
Is it ok to run different heads like this? Is it possible it actually left the factory like this?
Also,when I rebuild the engine, would I be better off using a set of 11 heads off a 1970 350 that I have?
The 11 heads were used on 70 400's as well, 1.96/1.66 with 8.6 cr listed on 70 400's.

Thanks for any help.
Mike