Just because your 305C has I beam manifolds doesn't mean there are differences in the head, these are still the same small port heads and exhaust manifolds. Lately I've noticed a lot of people thinking just because their V6 has I beam manifolds that they are Magnums. The fact is when GMC first introduced the V6 for 1960 there were no big port Magnum engines yet there were I beam shaped exhaust manifolds. Just a few minutes ago I got a message on youtube from someone saying I'm using 305 exhaust manifolds on my 478M,
well just because they're not I beam shaped and are a smooth casting.
I'm using 351E Magnum exhaust manifolds retain the proper look for a pickup truck. There were actually 5 different types of Magnum Exhaust manifolds.