Was the fuel sending unit 5/16 or 3/8 from the factory?

5/16. Only hemi cars got 3/8

Actually, the fuel line was 5/16", but the sending unit was 3/8". The factory just jammed the fuel hose on. The 1/4" return was used as well, with the vapor separator. I still have the original 3/8" sending unit from my A12 Bee, that I sold last year.

3/8" sender with a 1/4" return,,but the body has a 5/16" main line with a 1/4" return line

So they did use the 426/440 sending unit 2932962 ( 3/8 )
but used the 383 steel line which was 5/16?

Correct