I think the Monte Carlo tire width should be 6mm which conforms to the stock tires on most older cars and there are plenty of after market wheels with grub screws, inserts and tires. The 1970’s era cars with 3d chassis have room for 8mm tires, but they should run spacers to get to 6mm. Otherwise run a separate class for 60’s cars with narrow tires and early 70’s with wide tires.