slant 6 radiator on a 318

If the slant had factory air, you should have a full width radiator that cools similar to the stock V-8 one. Later cars had 26"W radiators, which sure helps (you didn't tell us your year). If an early 318 with the cast-iron water pump, the lower hose is on the driver's side so won't work with a slant radiator. The cooling you need depends on where you live and what you do with the car. Many cars overheat climbing out of the LA Basin going 60 mph steadily uphill in 110 F weather. Even a brand new factory design is barely up to that task. Add towing a trailer and smarter to go slow in 2nd gear.