I have an 85 Westy that had the same problem. After her head gasket fix we
took her to the gas station to fill it up for the very first time since we
bought her.This is when we discovered that loads of gas would gushout from
just behind the filler hole in the wheel well. Upon closer inspection we
discovered that the PO had unhooked all of the tubes that ran to and from
the overflow tanks and put *drywall screws* in them to cap them off. So
after hooking them all up again the problem only happened when I tried to
squeeze 17 gal. in....a listee suggested trying to top off the tank
incrementally...like put in 13 gal or so, go into the store and grab a
snack, come back out and slowly put the rest in, giving the tank time to
displace the air in it.It worked...I was able to  fill 'er up without wet
shoes. (Now I find why the PO screwed the overflow tubes...they were
dry-rotted and get squishy with gas if used).