Dave,
I checked your website and the 318-400 take a 5/16 sending unit, I mic my outlet tube tonight and it is 5/16 with no return. I know I'm a dumbass, but I didn't see any part number on it just the chrysler symbol. I have a 73 chally with a 340, so I think the 5/16 is the correct one. Do you stock it? My ring is good for now until I try and get it off.