Here's a tip from 1972 from Ford: slightly more important than max tire width or appearance is the need for both rear tires to be within 0.2" in circumference, as measured with a tire tape. If the rear tires vary by more than 0.2" (as the OEM Arrivas did), the limited slip clutches in your ZF will wear excessively while driving normally. And remember, the clutches are already 50 years old! Ford mentioned that sorting tires by circumference works well if the supply is large enough to find a pair within specs.
The LDS clutches are quite small in surface area and it doesn't take much to render a ZF limited slip inoperative. Using the OEM inflatable spare wore mine to uselessness in 50 miles! Low lube level will also affect clutch life. Repair is pricy as it requires ZF removal from the car and (likely) interstate shipping.! Rear tires around 26.5" OD will also allow an accurate stock speedometer & odometer, as well.