Vapor release w/ aftermarket tank? Need help

[Profish]

I put an after market tank under the bed in my 69 swb. I also installed an aftermarket fill tube that allows me to fill from the side instead of the top. The fill tube has a vent tube that I hooked up. The tank has two outlets other than the fill tube and supply tube.

Currently I am having a heck of a time getting the tank to accept fuel. If I leave the return line open, it accepts fuel, but produces major fumes. I originally had the return line plugged, but was vapor locking the tank. I've since ran a 1/4 line from the 1/2 inch return line on the tank, to a vapor canister that I got off a newer Suburban. I located the canister next to the battery. If I disconnect the line at the canister, I can smell and feel the fumes pushing through, but apparently it isn't enough in conjunction with the actual vent line, to relieve enough pressure to allow me to get fuel in.

I'm now wondering about cutting out the small diameter filler insert, to allow more space around the filler spout, for air to release. Is this likely to work or am I creating more problems?
Any thoughts are much appreciated. I can include photos if it would help.
Michael