I had three of these crack and fail on the motor that I know have in my 82. The first was the 79 oem unit, where the rubber died and the outer ring slipped. I installed a junkyard unit to drive for a couple weeks. The damaged key tore that up in 2 weeks. I installed a new replacement Ford unit. It lasted a year, and the bolt broke off, and it cracked the snout and fell off. Fortunately, the threads were ok, and the bolt just threaded out fine. The crank key slot was rounded at the top, but a new key seemed to fit ok.
I put the original 78 balancer for that crank on it, with a new key, and it has been there ever since.
However, I put a new Fluidamper on the next motor, and it has been fine for 4 years. The other motor will get a performance balancer soon.
I have always used a torque wrench, etc. Other than the first one just slipping, I do not know what caused it, other than guessing that the load and vibration of the old sytle ac compressor just lossened up the outer ring and caused it to shake and fail.