A thirty year old (or even fifteen!) spare is not worth carrying around, whether the special spare or a tire of equal age on a steely or slicer. You're asking for a blowout. I found one in a friend's car that had cracked and leaked out just hanging around for decades in the storage well. Never been used more than ten miles, but steel was hanging out.
For Gen I/II/III, I'd recommend, at a minimum find a tire that's the same average height as what you are running now (call it equal or a little lower than a full tread of your current tires) so in case you have to mount it on the front you will minimize the pull you will get to the side. Almost nobody I knew in the local SHO group was still using the original Ford spec 215/60-16 that was on the slicers. I had gone to 225/55 the first time I had to change all of them. As I went to wider wheels I changed sizes a couple of times more. If you're in a snow area, make the spare the same as one of your winter tires or at least an all season.
Anyway, best case is the
same exact tire you have on now. That minimizes the difference in braking, handling, etc. from side to side when the spare is installed. If you really want to wear them all out, and you have a tire that isn't directional, then you can rotate the spare through. If you are determined to keep a smaller or older tire as a spare, then if you have a front go down, move a rear to the front and put your "special" on the rear. Since a lot of tire lines don't hang around for more than three to five years, some cars may already have mixed pairs front to rear. A tire you buy now might have been made some time ago, complicating things even further; the higher performance the tire (Summer classifications for instance) the faster they age from their maximum performance. And even hiding in the garage does not keep them from deteriorating, though not as fast as if exposed to Sun and ozone.
Tire date codes:
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=11
Tire heights are available various places, I like Tirerack.com where you can put in the manufacturer, search the model, and see the size specs of mounted tires, at least for lines they carry.
Now, if you have a Gen IV or later all wheel drive, there are even limits on how different in height tires on different corners are supposed to be. Not sure of Ford's numbers, but on my Subaru they want them to be within 1/4 inch circumference which is less than 2/32 difference in height. I don't keep it quite that tight but have ended up changing a full set when I lost one to a rock. Also they really want you to have the same exact tire on all four.