Fuel sending unit questions

[jpayne79k20]

I have a 79 k20 with a single tank on passenger side. I replaced my fuel and sending unit. I replaced the 2 line sending unit with a new 2 line sending unit. I did fine a third line on my frame which is for the vent. So the person that had it before switch to a two line sending unit. My question with setup work or will the tank not vent through cap. I already had the two line in not seeing the third line on the frame. So if I have to change it I will.