Bill,
The easiest way to plumb a return line is to use an AN bulkhead adapter through the side of the tank. Pick your location and drill for the size fitting you want. Use a gasketed washer from an industrial supply like McMaster or Grainger. Here are two examples from McMaster;
Either gasket is rated to seal 1000 psi or more, so properly sized, there is not likely to be a leak.
An alternative approach depending on the internals of the filler neck extension, would be to drill and tap an o-ring boss port on the filler neck below the cap seal and above the bottom of the filler neck. Use an AN fitting size that would fit in the space available, probably a -8 or -10 (preferred). Put the appropriate o-ring on the fitting, for its size, and screw it into the neck.
Either way does not require welding.