You claim to have a 400, and most of the 400's indeed came factory equipped with it. So if yours was so-equipped and now no longer has it, I'd be curious why it was plugged and/or removed.
It was originally a 350 car, and the guy we bought it off put a Chevy 350 in. We put a 400 into it, so that's why there is no return line on the sending unit.
