I went with the late 90's 16 inch BBP Lincoln/CrownVic mini-spare (T145/80D16) for my 73 Duster. I had the front converted to modern disc brakes and the rear went with 15 inch adapters. It fit pretty nicely and I also got a Lincoln carpeted cover to go under the vinyl trunk liner. Search in the tires/wheels forum. There have been several threads.
I use a 17" spare from a '07/08 Infiniti G35. Fits in the well, it's not perfectly flush with the floor but it's pretty close. It clears my 13" rotors in the front too, which is why I had to use a 17" spare. I think it has a 145/80/17 on it (~26" tall). All I did was a little dremel work on the hub bore, it was a couple mm's too small. Nothing major though, especially for a spare.
There's a couple other options mentioned in this thread if you don't need a 17 incher...
''I use the well to store cleaning supplies.''
Good spot, just do not have a flat and be by yourself. I have a spare but no jack, so I'm no better off.