I wonder if there are still some good treads left further back in the crank snout, if one could make a stud, that will thread further into the crank than the stock bolt does, and then use a nut at the balancer? To be honest, I have never had any issues installing balancers on FE cranks over many years , although I always tapped the balancer on with a hammer, with a piece of wood or steel between the balancer and hammer. I have never used an aftermarket FE crankshaft, are the aftermarket cranks a bit larger OD, making them tighter fitting for the balancer? I know a local guy that built a FE stroker, with an aftermarket crank, not sure of the brand, and when he went to install the spacer behind the balancer, rather than being a hand slip fit, like factory stuff, he had to tap it on with a hammer, and it cracked the spacer, so I have to assume the snout of the crank was a bit larger, or there was a problem with how the non OE style crank key fit.