Filler neck interchange/fuel tank vent question

The hole in the trunk pan hole for the filler tube changed in 1972. I tried the same thing for my 1972. I wanted the 1970 18 gal tank versa the 1972 16 gal and thought I'd try the 1970 fill tube with the vent nipple. The vent tube goes down through the fill tube boot on the pan which also change in 72. That is where my problem is. The later boot doesn't fit the vent tube and the earlier boot doesn't fit the trunk pan.
All the filler tubes go in from the inside.
Also H/T, Dart, filler tube has 4 screws to the quarter and F/B, Duster, has three screws.
Still don't know what action I'm taking next.