why would my 225-75-15 tire rub on the frame

And neither of those sizes is correct! Back in 1970, I believe these trucks used a 14 inch tire, and the correct size, with the conversion, would be a 225/75R14. If they were originally a 15 inch rim, the tires would be a GR/78R15, which translates into a 205/75R15. I would go with a 225/70R15, and you should be just fine. If, however, it was originally a 14 inch rim, you will want a 225/60R15, and that will keep the tires from rubbing the frame.

I went to several places to try and get the right size for this vehicle and boy did it stump the tire guys. Michael your pretty right on about the sizes because their conclusion was similar to yours. It's really hard to find info. on this gal. The door sticker is long gone unfortunatly. I'll try the 225/70/15 , thanks for your help.

I was buying tires for cars that had that very conversion as a factory option back in the day! In fact, with a little luck I will soon be the proud owner of a 1980 Colony Park wagon, which had 225/75R14 tires standard, but a 205/75R15 as an option. My 84 Delta 88 had 205/75R15s, and a 225/70R15 was just right. My 78 Thunderbird came with GR/78-15 tires as standard, and that converted to a 225/70R15 just fine. Mix all that up into a stew, and we end up with a fairly easy conversion for your Ranchero!