kelsey hayes vs f/m/j swap

The K-H disk brakes are great. They were used on high-end Mustangs the same years too. The fixed 4-piston calipers is considered better than sliding calipers, and preferred by racers. A member here (near me) rebuilds the K-H brake systems as a sideline. You can also get rebuilt calipers from rockauto and new disks are now very reasonable (Centric brand).

I don't understand why your uncle thought those brakes couldn't stop the car. The ultimate limit is when the tires skid and even 9" drum brakes can easily skid the tires. The advantage of bigger drums is they can absorb more heat, so can handle higher-speed braking better. The advantage of disks is that they cool faster, especially those w/ internal passages and fins that act like a fan. That allows quicker recovery for things like constant braking in road-course racing. Front disks were mandated by the feds ~1973 because too many drivers didn't understand how to use the engine to brake the car on downhill runs, thus overheating the brakes and going off cliffs. Disks can give more even braking because drum brakes are more sensitive to changes in friction such as from oil or leaking brake fluid. Disks don't have the amplifying effect of drum shoes, so take more force on the pedal, thus vacuum boosters are more common though many A owners say they aren't necessary.