1977 kz650 Tyre sizes

hey riders, i ran into an issue a few days ago, i noticed the rear end would get squirrel-y in the corners like the rear tyre side wall was collapsing. After inspecting the axle and tyre pressure and hub adjusters i found out the PO PUT A FRONT TIRE, ON THE REAR WHEEL. the blaspheme. but im wondering what is the absolute widest tyre i can fit on a 1977 kz650b rear spoke rim without rubbing the chain. it also has the chain guard removed if that helps. cheers!

Probably a 130 but that would be too wide for the rim and the tire shape will distort just enough to make the rear a little squirelly in corners. A 120 will give better handling in my experience (I've had both on my 550's which use almost the same if not the same rim sizes). Some might even argue for a 110.