I really don't seem to have all of the equation to beable to help.
I think I'm missing something...
First post I thought the way I understand it was that he wanted to put in a propane powered Small Block Chevy. (yes/no)
But then the second post says he wants to put in a Cummins. (Yes/No)
For the SBC you can get kits that include the engine mounts, hogs head adapter plate, carrier bearing relocation, wiring harness, etc, etc. Everything that you will need to drop it in and fire it up, all in one box.
For the propane; that conversion comes in a kit too. Installed with simple hand tools, I think if I remember right, a hand Drill was the only power needed.
On the diesel, First off: I am not sure if the T18 will handle the torque of a cummins or any diesel for very long. Second I doubt a production adapter will be available any where. That wil have to be custom fabricated at a machine shop. Also make sure there is enough room in the engine bay for a diesel and radiator. A diesel requires a lot bigger radiator to keep it cool because diesel burns much hotter than gas. Also that may require lots of fabrication, firewall alterations, custom engine and transmission mounts for proper drive shaft alignment and suspension articulation. Also major suspension upgrades to the front suspension to handle the added weight. I've never attempted such a project myself. But I forsee many, many, many, many, unforseen obstacles in this energetic of a project. Bell housing adapter is going to be the easiest part!