Can I use '70 vinyl upholstery on a '73 seat, or will I end up with upholstery that doesn't fit my current seats.
Unfortunately, it is not quite that easy. The frames in the seatbacks are very different.
In the first 2 pictures below, you can see some 73 seats that I took apart. On the left is the way the seat looks with foam and cover, and on the right is the steel 'shell', after I tore off most of the foam. They apparently formed the foam right onto that shell, because it is
tough to remove.
The 70/71 seats have a traditional wire frame with springs (instead of this 'shell'), and the foam slides over it. I'm sorry I don't have a picture of the frame, but you can see some finished 70 seats in the 3rd picture below. The shape is somewhat different than the later seats, and the 70/71 frames have an open back, which is why they have that plastic cover.
In addition, the 70/71 have a button on the side to flip the seatback forward, while the later seats have a lever on the back of the seat. (You can see it at the lower left of the 1st picture).
As far as I know, the seat bottoms are the same for all years.
Having said all that, I would talk to Legendary, as Barry suggested. You can find 70/71 frames, but they aren't common (I paid about $200 for cores...) Best of luck to you!
-Tom