Rear tires for 65 Dart

Good luck putting 225s on 8s and then getting the tread flat to the road......
225/60s should go on 7s.
60 series on 8s should be 255s.
But if your backspace ain't just right on the money, I doubt you can fit 8s into the wheelwells.

So mount the rims on the drums without tires and start measuring.