Help with relocated gas tank please.

[69chevytruck]

I relocated my gas tank to under the rear of the bed using a new blazer tank. But the problem is that I am unable to fill up the tank because the nozzle shuts off and gas spills back out of the fill tube. I tried the fill tube (1)running out to the side of the truck, (2)over to the wheel well and then up, and (3)then venting the tank using one of the other connections on the tank, but nothing helps. The tank has connections for using a charcoal canister and a return line I have a fill tube attached to the tank over the tank inlet.

Any Ideas? I don't want the filler in the bed because it is a truck and use it as a truck.

And if this has been discussed somewhere else on the board, can you direct me to the right place.