1978 Suburban fuel fill issue

[Advanced Design]

Guys, appreciate your ideas on this one. A buddy just completed an LS swap on this 78 Burb. Filling the fuel tank is very slow and difficult. Condition existed before the swap. Still exists with the vent disconnected at the tank. Gas just backs up in the fill pipe.

I've searched for ideas here and I did find several threads about the issue on trucks with no good solutions.

We figure it is the small diameter of the fill pipe coupled with the very slight down angle that causes the fuel to back up. Tanks inc has a larger diameter tank neck that we will try.

Unless there is something I am missing? Appreciate your ideas.