Assuming you can separate the two parts you could always make a gasket,failing that i'd imagine any good quality gas resistant RTV sealant would suffice.
I have a similar issue on my repro (in cab) gas tank but as you've noticed the neck is actually welded at that point on the repops and on mine the weld is porous.
The original in cab one had a bolted two piece assembly much like the 47s under bed tank.
Rather than pull the tank and get it welded (with all the risks that involves) i'm going to wrap the join with fibreglass mat and resin,hopefully this will cure the smell in the cab.
It's only evident on fill ups and tight left hand corners depending on how much gas is in it...and how hard i'm driving it.
