To resurrect and add to this thread, I had the same issue and here's my solution.
My E3 is a 1970 2500. I replaced the tank with a 3.0Si tank, presumably from a later model. It would seem that the later models have some additional bracing in the rear passenger quarter that changed the shape of the tank. The later tank is deeper and narrower, but it bolts into the earlier cars with most of the screw holes reused; it just leaves a sizable gap on the right. I filled that gap with a filler plate that I made. See below.
Hope that helps clear up any confusion.