I also bought a donut wheel/tire for my 74 Challenger. I still have 14" wheels with 235x60x14 inch tires.
I measured the backspacing and searched the pull-it-yourself salvage yard until I found a 15" donut and like new tire, that had our 5 lug 4 1/2 inch bolt pattern, and proper backspacing. Yes the 15" donut had the same nearly the same tire height as my 14" 235x60 tires.
I also had to grind a little of the center hole, to fit on the front wheels, and no problem for the rears. I think it came off of a Dodge Mini-van around a 2004 time frame. Not sure... Didn't really care.. I think is was $20.
Also, I was missing the jack and lug nut wrench. Hate bumper jacks, so found a Scissors Jack and folding lug nut wrench. $9 dollars, and I put them into a nice black bag I bought. Oh I also had to get (from salvage yard) 5 standard lug nuts, as the cones that I have for my normal wheels wouldn't work. Salvage yard thru them in for free !!!