need advice- TH400 extension housing seal

[brutus]

I'm replacing the extension housing on my '72 C20 because it is cracked. The replacement housing I got has an O-ring groove on the mating surface. I've been looking allover and can't find any reference to the TH400 short tailshaft housing taking a large O-ring to seal, only the thin paper gasket which I already have in my seal kit. Other than that the housings look identical- both large "C" castings.

Anyone have any experience with this? Did GM switch to an O-ring seal on newer TH400? If so, anyone have the part # on it.

Thanks