I've seen a number of them done with a 5.9, both 12v and 24v. They don't fit under the floor like the original 6 banger did as they are taller/longer, so best bet is to mount behind the cab. If you need measurements, I have one sitting in my shop.
Anyone know if Willysbill's site is gone or has it moved again. It had a lot of pictures of how others have done stuff.
Russ, vet57air on here has a ***mins under the seat of his 54 Tourlinere, pics can be found here https://www.flickr.com/photos/90901505@N04/sets/72157632203097752/ One of the nicest builds on this or any other COE board.
Had to move engine back 7"and down 3" using 95 dodge 1 ton ch***is. Also used a industrial manifold which moved turbo to the side and down. IMG_5547 - Copy by Pleasantleafarm posted Mar 10, 2015 at 9:55 PM
Here's a link to Willysbill's website. http://coetrucks.atwebpages.com/index.3b.html Lots of interesting builds there
Have a look over at the Garage Journal Forum (belongs to the HAMB) > https://www.garagejournal.com/forum/showthread.php?t=403069