Your original sending unit "vent" is a vent, as you stated. On G-bodies with gas, there's only TWO types of sending units. A two-port and three-port. The two-port has a charcoal canister vent and a fuel supply to the pump. In the three-port, they added a return-line from the pump. Depending on which type of fuel pump was used, that determined which sending unit you got. And that seemed to change year over year. I really don't know what the criteria was. Oldsmobile V8s used the three-port a lot. Chevies, not so much. The part numbers are only 1 different than each other as well. 25001612, the two-port, is what you have and 25001613 is the three-port. The last number just happens to coincide with the number of ports for A/G47. Wagons are similar, but obviously different part numbers (25000757 for 2 port and 25000758 for 3 port).
There is an actual RPO for the fuel line return. It's K15. Although, I do not know what triggers it. ALL 307 VIN 9's use it because ALL 307 VIN 9 engines came with the same type of fuel pump which has a return line.
Looking at john87442 crusty 442 frame below, you can see the vent, return, and supply. All G-bodies should have at least the vent and supply. The "heat shield" plate below the supply/return lines in the pic is only found on VIN 9 cars with the dual exhaust. Single exhaust cars didn't get those.
View attachment 215093
This portion below shows where the lines route to on the forward part of the frame. Where the canister is on the passenger side. When the canister is on the driver side, you should see the vent line run right up the driver frame rail in front.
View attachment 215094
Thanks for the refresher sir, very much appreciated. I knew the vent line went to the charcoal canister, but was unsure about the third line as my 305 equipped cars never had the return line from the fuel pump.
I’ve repurposed the stock 2 line pickup the car originally came with now last night into a dedicated fuel level sending unit with just a vent.
It’s Mike’s day to help out here today, so one of the first things we’re going to tackle is this bullshit:
Frigging aftermarket Spectra tank, I can’t even attach the new gas cap until we readjust the filler neck angle.
Picked up the newer style factory replacement cap you linked to.
After that, we’re going to drop the tank, check out what needs to be done to tweak the float arm to clear the Aeromotive pump and hanger, then get it installed in the tank. The Summit order arrived Friday, so we’ll also be looking at making the wiring connections with the new pump wiring harness.