Thanks for the additional information.
Wagons have always been problematic as to fuel tank issues. Due to where their tanks are placed. If the rear seat was not there, it would still be an underfloor area for storage and such. Their lower production rates and later attrition rates also play into their potential replacement market for fuel tanks, unfortunately.
The one good thing is that all C-body wagons, within a platform model year range, all should interchange. 65-68, 69 and up, for example. Unfortunately, still not a lot of wagons compared to sedans/hardtops. Especially in the expanses of land west of the I-35 corridor.
Still, there are some larger vintage vehicle salvage yards still around. I don't recall any C/Slab wagons at Browne's in Sunset when we were up there last spring. Also a larger yard between Graham and Breckenridge (closer to Graham), plus the larger yards in the Lubbock and Amarillo areas. Some larger yards in OK that do older cars, too. Browne's is apparently in a nationwide circuit of larger older vehicle salvage yards.
I'm suspecting your tank is not repairable?
Just some thoughts,
CBODY67