the vent on the filler stem is what allows the air in the tank to be pushed out when filling,
so you need a wide tube that'll allow air to pass while filling or you need that vent. if you got a big fat hole and tube that the fuel nozzle doesn't block, and the tank eats the fuel without hesitation, your moving enough air.
if you use a nozzle size trap door flapper like OE, then you need that vent tube, there is no room for air to pass and escape.
1.25" is the problem. it's the small end of the spectrum and not always easy to find items.