I do NOT mean to insult you, but I don't know how much experience with wheels/tires you have.
Assuming no tires on the rims to tell you what size they are, measure across the front of the wheel, at the widest part . If you get 15"+, those are 14"wheels. If you get 16" plus, those are 15s, and unicorns!
My 67 273 barracuda has 14x4.5 stock. The /6 cars are mostly 13s.
Edit: Heck, even the B-bodies in the sixties only came with big pattern wheels, and15s ONLY on Hemis and sixpacks. Everything else got fourteen!
2nd edit. Yes you are measuring the width properly, between the beads.