There are a couple different options for diesel. The 6.6 duramax is the best overall winner thou.
You can do the 5.9 cummins.. Lots of work
The babymax is what I would do if I had the shop and know how.. Not been done before
3.9 4BT
www.4btswaps.com
And the 5.0 Cummins V8. But to my knowledge it hasnt been done.
The 6.5 was a decent motor... But not good in any way.
The 6.5TD were prone to crank sheering
The 6.5NA were prone to EGR issues
Both were very prone to PMD failure... That is a very expensive fix!
I had a 6.5NA that just would not die. powerful enough for what it was... Always rolled smoke no matter what you did really. I put 90k miles on mine in 1 year, and the only issue I ever had was the EGR. They rely heavily on a vacuum pump that are very prone to failure.