Gaskets for a C20/K10?

[Corey'sgotachev]

My 70 CustomCamper/20 really has a 350-SM465-NP205. I have a growing puddle of oil under it. It seems to be mostly dripping off the cross member/transfer case mount. Oil is engine oil (vs gear oil).

Trying to determine where the heck the oil is coming from. Do the transmission and transfer case use gear oil?

I am thinking the rear main seal is leaking but not 100% sure.

In trying to "fix" easy things first, like valve cover gaskets and oil pan gaskets, what should i replace them with?

Thanks non-leaking Chevy Fans!