Will wheels from a 1974 Comet fit a 1966 Mustang?
Maybe. If your car has the factory 289, you should have 5 lug wheels. Comet wheels are also 5 lugs and have the same 4 1/2" lug spacing. Factory 1974 Comet wheels were 14" in diameter. Most folks prefer to put 15" wheels on their Mustangs. However if you found just some sort of wheels that you like that happen to be bolted on to the Comet, then sure, they'll fit.
However, if your car was originally a six cylinder Mustang and has 4 lug wheels, no. 1970's Comets all had 5 lug wheel patterns.
If you are interested in changing your car over to 5 lugs, that 1974 Comet would likely be an excellent donor for the entire rear end. If it is an eight inch rear, it's almost a perfect match for 1965-6 Mustang V8 rear ends. I have a 1974 Maverick/Comet rear end under my Mustang right now.