Sending unit?
 

hello, I have a 1966 chrysler newport. All I need is a sending unit as I am looking for them I have found 2 and the only difference is one is 3/8ths fuel line and one is 5/16ths fuel line I was wondering if there is that much of a difference or which one I should buy

It depends on what engine you have ? With a big Chrysler I would almost guarantee it is a Big Block and should have a 3/8" fuel setup

i had a 300 L 413 engine 360 hp. ran fine on 5/16 line. and most of the 440s late 60s used a 5/16 line. i think in the 390 hp range. if i remember rightly.