Ah, much easier to build than the mount I'm running, My current mount is simply a 3/8" cube with the faces rotated to meet the block and subframe. My first CNC project
Might just section the subframe and lay in a section of 3/8" plate for the top section to lessen the chance of fatigue, then grab another stock '93 mount and drill it per your link.
What sort rod did you end up with? I'd assume 1/2" grade 8 bolt would do the job, assuming it had rolled threads. The cut threads on allthread leave all sorts of stress risers, as you saw with your second version.
Have you noticed increased wear to the dogbone coming off the diff yet? It seems that the rear drivers' mount ends up taking a bunch of force, then the dogbone ends up taking some too.