I just installed a 22 gallon tank from a 70 Mustang into my 65 FB.  this is how I modified the filler neck to fit the new tank.  I made a cardboard pattern by tracing around the filler neck:

In the above pattern, I removed a couple of wedges from two different parts of the tank end (patched with blue tape), until I concluded that a 1" slice from the other end would result in the best alignment with the inlet to the tank.  The next picture shows how I made the cut:

The area that must be cut is 1" wide, which corresponds to the blue tape.  I drew a line across the top of the tape to mark the bottom and top pieces so they could be realigned.  After cutting out the blue tape with a hacksaw, I positioned the filler neckon the tank and noted that the fit was very close, but was improved if I rotated the botton piece about 1/8" from its alignment mark.  Here's the finished piece after I welded it back together and ground down the weld joint:

You can see the cut out piece.  Some observations:

(1) Just cutting a piece from the bottom may work, but won't result in the best alignment because the curvature of the neck changes along it length.  It's also bent slightly to one side (at least mine was).
(2) The filler neck is made from welded seamed tubing that is pretty thin (thinner than the 19 gauge used in much of car), so use a low heat when welding.  I used "C2" in my Lincoln 140 MIG welder with 0.030" wire, but 0.023" wire would have been better.  Of course TIG would have been optimal.
(3) Like replacing floors or panels, it's better to tack weld around the perimeter of the neck, gradually filling in the space between tacks than to try to run a continuous bead.
(4) My neck had some varnish inside of it.  It's easier to clean the inside of the neck before welding the pieces back together.
(5) I used a stone on a drill extension to smooth down the weld on the inside of the neck.