depends what motor was in the mustang and what motor is in the comet, if they both have 6cyl. engines I would say there's a good chance it will work, if you have a V8 in your comet and the mustang radiator was a 6 cyl. it won't work due to being too small to keep the V8 cool.
If you have the measurements, you can tell by yourself. The main things were the position of hose connections and whether or not it had transmission cooling connections. The heights were pretty much the same.
Most of the time compatible radiators went TO Mustangs, not from them to others. I know the '70s Comet and Mavericks lost a lot of radiators to Mustangs.