When it comes to strictly engine parts, my local Detroit dealer doesn't have any issue getting a hold of them. Its the parts of the truck that are special to the Detroit (or General Motors Diesel, I suppose) powered 450s that are near impossible to find. The engine mounts appear to be a GM part to adapt the engine to the chassis. For now I have sourced some vibration isolator mounts rated for 720 lbs max per mount from Acklands. Ill have to modify them a little bit, but for now its my best option I think.

Here is a link with a bunch more pictures, dont mind the random pictures of close ups of components.

https://share.icloud.com/photos/0dOK-m0zNzRKc3AXXrmcX1JlA

Here is a couple of pictures of the rear mounts

https://share.icloud.com/photos/0Fenr37_1bI6hP6UKjm8ZF-1A