3 outlet VS 2 ooutlet fuel sending unit question

[1972BackInBlackC10]

i have a 1980 gmc that im replacing the fuel tank in, the truck has dual tanks but im only replacing the pass side tank and not running the driver side, anyways i want to put a new sending unit in and theres one with 2 outlets and one with 3 outlets, what sending unit do i need if im only running the passnger side tank?

i thought that maybe the 3 outlet was used on dual tank trucks and the 2 outlet is for single tank trucks, but from what iv read im starting to think the third outlet is for a vent line? can anyone help me out here???