Fuel Filling Question

I have a problem putting gas in my Valiant. From empty it will only take around 3 or 4 gallons and then it will start overflowing out the fill tube and down the side of my quarter panel. I havent pulled the filler tube and tank yet as I need a new sending unit (guage doesn't work) so I figured I would take care of it all at one time. I know a lot of the A bodies had a slow fill problem but this will take no more than 4 gallons. Any ideas.
Thanks in advance,
Paul