ps- 1969 440 cars SHOULD be a 5/16" main line and fuel pump inlet + outlet, BUT with a 3/8" sender at the tank and with a 1/4" return line.

3/8" main lines in 1969 SHOULD be Hemi only.

I've owned 4 1969 GTX's and none were 3/8" main line.

again, I'm not saying all were 5/16" main as I know of a few that are supposed original 440 cars with 3/8" main lines