With no other viable options at the time, I made the following upgrades to a 2010 Mac Pro (5,1) about two years ago - primarily for rendering with VRay. VRay uses CUDA, so the AMD GPUs of the trashcan Mac were of little use and adding Thunderbolt chassis for external GPUs would have just added...
OK, so a while a go I stumbled on basically this question asked on a forum...
"Can you put a G5 internals into a G4 case?"
So, can you?
Well, yes!
You can either extend the case several inches, which is very difficult and very time consuming.
Or simply trim off the excess motherboard length...