Had posted this else where in a tank discussion, but if you do use the Elco tank in a 68-72, look at the filler/vent tube assemblies used on Chevy/GMC G-Vans (early 70's-95?) and Chevy Suburbans (73-early 90's). These use a long vertical tube with a bend connected to a horizontal tube that leads to the tank. All connected by short lengths of rubber hose. May be more easily adapted than trying to make from scratch.
Perhaps a solution, but not an easy one. The gasoline filler doors on the wagon and the pick-em up are in totally different locations. It would take some creative plumbing to modify the Elky filler neck to match up with the wagon filler door and possibly some sheet metal work to accommodate the filler neck modification. Also, fuel line(s) would have to be rerouted along with sending unit wiring. But then, if you are going to go to those lengths out of necessity, I suppose it would even be worth considering eliminating the foot well on a 3 seat wagon to accommodate the Elky tank.Elky tank wouldn't fit on a 3 seat wagon obviously but is a solution on the 6 pass ones.
Really would like to see someplace start producing gas tanks for these cars...wagon popularity is growing.