Fuel Tank relocate sending unit connections

[oldgmc]

I usually work with older vehicles 50's,60's so need some help on this. I installed a rear tank from 75 blazer and will be removing the dual tanks. I have the new tank in but as the connections seem a little different then the original I need a education on how this works. The new unit has 3 ports the one to the right is easy that the fuel feed. I have 2 lines going to the fuel pump so of the 2 remaining options which one goes to the other fuel pump line the small one with the valve or the large one ?. I am guessing the small one with the valve but not sure.
Why 2 lines to a fuel pump
The truck is a 1979 GMC K35 , 350 engine , automatic (th400) with a carburetor and is a 1 ton (heavy emissions) so no canisters, egrs, cats or other emissions crap. See photos below
Thanks Ed

1st three are of new unit last is how old tank was set up