A 16 x 6:50 x 8 is a front tire, not a rear tire.
The last number states the rim size, which is 8 inches in diameter.
The middle number relates to the width of the carcass and the last number (16) relates to the measurement from one edge of the rim to the other edge of the rim when the measuring tape is placed across the tire.
The rear tires on a Case or Ingersoll have a 16" rim diameter. Your tires should be labelled 8:00 x 16. Good used tires show up on e-Bay almost every day with and without rims. Why do you need tires? I have tractors that were built in the sixties and they still have the original rear tires. Yes, sometimes dry rot can be a problem but tubes will often cure that problem unless the rot has extended to cord separation. I don't think I've ever seen a bald turf tire on a Hi Wheel tractor.