1980 gmc vented gas cap?

[1972BackInBlackC10]

would a 1980 gmc have a vented gas cap or would it have a vent line on the sending unit? i inquired about a sending unit at the parts store, they phoned their tech line and the guy told them since im running one tank that i need the sening unit with 2 outlets, my tanks has a sending unit with 3 outlets

2 lines from the fuel pump run to the sending unit and the 3rd line has a volt shoved in the end of it, i was assuming one line was for fuel from the tank to the fuel pump and the second was a return line from the pump and the third is my vent liine does this sound right? my truck does not have a charcoal canister, it did have dual tank but im only putting one 20 gallon tank in, what sending unit do i need the 3 or 2 outlet? please help i need to get my truck on the road iv been working n this thing for a year now i need to get it driving