The Stovebolt Site Forums The Parking Lot Big Bolts Information for a GMC 450

I searched "repair broken truck motor mounts" and this came up:



along with several others.

There appears to be some good information there, more in the comments.

A mold could be made of sheet plastic or tin, whatever worked and could be taped in place. Excess could be cut or sanded away.

I don't know this guy and I have never tried this method but my experience tells me it should work if you can't find another solution.
Here's hoping you get a good repair one way or the other.