I have a friend building a 72 Scout II. He wants a diesel, and we've been round and round on which one. The easiest to get is a 617 obviously, and with me as his friend all he needs to do is ask and I'll find one for him. But with a large engine bay there are may options for him to choose from. My dad is in the midst of swapping out his 7.3IDI with banks kit from his 88 Ford, My friend is considering that swap, but the weigh it much more then the existing 345. We've come to the conclusion that the 4BT is the best option, but he doesn't have the funds at the moment to buy any of the ones we've found for sale.
I honestly think the 617 could be a good match, at least for what hes using the scout for. He started a 4WD exploration club. Once a year we take a week or so and explore ghost town in Nevada. With that concept in mind fuel efficiency and mechanical simplicity, as well as parts availability are just as important as HP and TQ numbers. There is a fellow here on this forum who is mating a 617 to a transmission for a Toyota truck for his Landcruiser project. This is the route I suggested to my friend. I'd add to that a dual transfer-case setup from Marlin Crawler and I don't think you'd have any problems with torque. Not saying you'd go anywhere fast. But that's not really the point.
Best of luck with your project. I already sent a link to this tread to my friend. I'm sure we'll both be reading as you make progress!