With most projects, the actual work will be much greater then expected. There is a good amount of information on this site regarding putting in the 6.2. The search feature should find them. Many of the posts were by me when I put one in my own M715. For example, you will need to find a flywheel from a 6.2/6.5 w/ manual tranny. Other flywheels from Chevy gas engines, may bolt up but are not balanced correctly. You will need to fabricate new clutch linkage even if the 6-cylinder already has it done as it will conflict with the oil cooling lines coming out of the side of the block.

As others have stated, the best choice may be to run the engine you already have in it.

If your heart is set on a diesel and you already have the Chevy adapter, the 6.2/6.5 is very doable.

All other diesel choices mentioned in this post will require so much work, that most people in your situation would abandon the whole project before successfully making the swap.

Just my quick opinion,
Rollie