Just found a forum off of Destroked.com. Someone on there is trying the 5.9L Cummins conversion into his F150 and found it too much of a hassle... too much power for the 1/2-ton. Money would have to be dumped into beefing suspension and axles. Basically, this is a project better suited to a 3/4-ton or bigger. Instead he's going for a smaller 4-cyl. Cummins.
I also have a '91 Ranger. Recently took the engine (2.3L) out of it. My current plan is a 4.0L swap. Know of any small diesels that would do good in this project?