What weight oil should I use - Brand?

[38 coupe]

You have the correct fitting, it is the one the oil pressure sending unit is threaded into.
Your engine appears to be assembled out of several sources. Your car left the factory with a tangerine orange engine with EAB heads. The intake looks like a faded/worn version of tangerine orange. If everything is working I would leave well enough alone. Several times I have learned, the hard way, that if "it ain't broke, don't fix it".