You can check LMC truck, but I'm not sure if they have for that year.

Which filler neck is that -- the one from behind the driver's door area?

On my 73 d200 I just made something recently to replace that filler tube. I got about 8" of the 2" fuel tube from Carquest, and used two 90 degree 2" exhaust bends from Autozone. Finally, one 1 1/4" iron 90 degree pipe from lowes. Cobbled it together with pipe clamps and welding the exhaust bends together, and it works great. If helpful I can take a pic.