Will this 7-blade fan fit my '66 Dart Slant Six?

As much as I dislike them, I think I'd go with a fan clutch system over the ridgid one for an A/C setup. You're going to need all the horsepower you can get with that 170! Measure the distance from the center of your water pump shaft, out to the nearest obstruction.....usually a radiator hose. Double that and subtract an inch and a half. That will be the maximum diameter you can use safely. Buy a fan clutch that will hold the fan blade an inch to an inch and a half from the radiator core. I would recommend a shroud also.