you can turn it into a 20 gal tank, there's a writeup somwhere on this site i think. there's a tube on the overflow of the tank that goes in about 3'' (i think, can't remember exactly), removing that will turn your tank into a 20 gal and probably fix your filling issue.
the other thing is the pumps I have seen (and that is valid for both the Jeep and the Camaro i have) are "clicking" full when you squeeze the handle all the way, very few don't. Most of the ones that have the hold feature (they removed that in BC/Canada btw so i only see it in the US whenever i go to fill up with cheap gas) would auto-stop right away if i set them to max, if i set the handle to 1/2 would fill up no problem (but takes twice as long), but this is not overflow related but the small fillneck on the tank as it won't be able to bleed fast enough and would trigger the stop on the pump.