‘66 Valiant tire size?
The correct replacement tire for your car is P185/80R13 but they're hard to find. Do not use a trailer tire.
Maxxis still has these tires and they're a good deal direct from them:
Maxxis P185/80R13 90S RWW MA-1
If you're thinking about going to larger tires, see
Tire Upgrades.