I was talking to a friend that restores cars and he said that most of the big block solid lifter Camaros he has seen, did not have the bell housing painted. He did say that on all the Corvettes they were though. The Camaro Z/28's were painted also. But there are too many inconsistencies to say which is correct. Back them there were no standards to go by like there are now.
So I guess it can go either way and be considered "correct".