Very little will work with the slow geared 2 speed rear and todays speeds on the highway, perhaps not even your gas engine. Figure speeds and RPM here
http://www.onlineconversion.com/bigger_tires.htmIf you want a diesel that is a relatively easy swap the 4BT Cummins (2/3 of a dodge engine) should work pretty well. I think plenty of power, but won't be a hot rod in a 2 ton truck.
Have a look here
http://www.4btswaps.comTo try to overcome the poor rear axle gears a six speed Allison 1000 or 2000 automatic transmission with the about 0.65 OD ratio in high will help. Or if manual is a must an NV4500 and a us gear or gear vendors OD unit. Run the numbers, an axle/gear change may be necessary after all, or just drive slow.
If you desire even more power and speed, even with the gas engine idea, you might want to start looking at brake and perhaps even steering upgrades too.
As for a larger diesel, the 6BT comes to mind but I doubt it's worth all the effort to make it fit. Instead I'd start with the 4BT and make some performance upgrades.
In the end you should do what makes you happy, it's your truck.
Grigg
