My tank for the 68 fit like a glove.
There are two guys at Vans from what I understand. One is very nice and helpful. The other has been spoke of above.
There was a guy previously that compared the parts from a 70's 2 wheel dr dodge pick up for doing the disc brake swap and it seemed that it was very close. If I'm remembering correctly he had to put a 1/4" spacer in somewhere.
I bought both my tank and sender from Vans, 8 or 9 years ago. The sending unit would not read fully than 3/4 of tank initially. I tweaked the float arm down the slightest bit and it indicates much better now. Use your old lock ring.
Does it have a dual heater core like the one pictured? And a "h" like is pictured below? The purpose of that valve is to let some hot water to flow so the evaporator doesn't freeze up. Maybe it is letting too much flow.