A 383 Challenger R/T was not available with either F60 tires OR a full size 15-inch spare. The correct tire size for a Challenger R/T 383 equipped with 15-inch wheels (which is incredibly rare in and of itself, they are normally 14-inch wheels) is E60 15.
Your car should have a collapsible spare or possibly a 14-inch full size spare. Your ad says "upgraded" to 15 inch wheels, so I guess it did indeed have 14s? Have you found a broadcast sheet for the car?
What you have there is a combination of wheel and tire that is probably out of a B-Body: Roadrunner, GTX, Super Bee etc. and is worth about $1000-$1500 to the right person as it has the rare '450' steel wheel.
OR F70-14 full size spare:
