I will give it another try with minimal insertion and just a dribble LOL.
I guess we should clarify though, that while your problem is an unfortunately all too common one, it's still not right.
You should be able to fill your Bronco like a normal human being without having to resort to trickery.
Well, I say "normal" but that would indicate you could just stick the nozzle in, set it and walk away. But that ain't gonna happen!
You should however be able to stick it most of the way in, pull back on the boot and fill the tank fairly quickly.
So yes, you still have something wrong. What it is, we don't know yet.
My '71 did it from the day I bought it (both tanks) until the day, several years later, that I changed the hoses and blew out the vent lines. No rhyme or reason to it, but it worked just fine after that.
It continued to work just fine when I installed my 23 gallon tank too.
My '68 is back to the old tricks though, and with this 23 gallon setup I can just barely dribble it in. And even still, it will back up and flood out of the filler neck when I'm not watching. Very frustrating to see the old telltale reverse-V shaped dribble permanently marking it's place in my new paint.
If you're older and noticed that kind of thing, that mark in the paint was an extremely common fixture of old cars and trucks. Especially trucks. I used to think it was just careless people because all of our cars always filled easily and even if something spilled, all of our cars just happened to have hidden caps so no paint damage was possible. Guess my dad was smarter than I thought!
After buying my first Bronco I realized that they weren't all careless after all!
Ya just gotta' walk in their shoes for awhile...
Paul