Underbed gas tank very slow to fill. Any ideas for venting, etc?

[Old Yeller 1970]

Way back before I restored Old Yeller I removed the cab gas tank and put a Blazer tank under the bed. Since I have a step side I routed the donor Blazer's fill neck out the rear of the left fender. IIRC, the tank came off a 71-72 body but was missing the gas cap. The 70 cap from my in cab tank didn't fit. When I bought a new cap I ordered a 71 cap but it didn't fit either. Ordering a 72 worked and has been on there ever since. (Who would have thought three different years would have three different gas caps?)

The only issue with it I've never resolved satisfactorily (this has been going on for years) is filling the tank is slower than pond water. I have to hold the nozzle sideways and at just the right angle and barely squeeze the handle to let the fuel tickle in. Putting the handle on the slowest setting usually is too fast and it will back flow out the inlet.

There are no restrictions in the fill line. There is one vent line that comes from the tank up to the fill neck. It has no restrictions in it. During the restoration I replaced the OEM tank with a poly tank. The poly tank has extra ports for hoses etc and I added an extra hose barb and hose and ran it up inside the fender next to the fill neck, put a bug screen over it and tied it off so it vents directly to air. I'm not at all satisfied with it being like that. It did help some but not much. It's possible the angle of the fill neck on Old Yeller is at just enough of a different angle than a Blazer to cause my problem. It doesn't look like it should be but I can't rule it out.

Reckon I'm going to have to live with it or is there hope for me?

Here's what it looked like before the resto and adding the second vent line.