putting a Cummins into a big Wagoneer?

This has been discussed ad nauseam here. The six cylinders from over the road trucks are probably too big, but a four might fit, if you can get your hands on one. UPS and other users of mid-sized diesels rebuild them and rebuild them, until they can't be rebuilt any more, then send them back to the manufacturer, who magically reconditions the blocks, for another million miles or so of use. Opportunities for a junkyard shopper to "pick one up" in usable condition are few and far between.

A Cummins from a diesel pickup, maybe. If you can get your hands on one, the engine cost is likely to exceed the value of your Wagoneer.

Earle

The 866 cid Cummins has no four cylinder version, but if it did, it sure as hell would not fit, as it would be the same size as the 4-71 or Series 50 Detroits.

Medium and light duty diesels are not in terribly short supply. The commonest is the five cylinder Benz, but Isuzu, Mits, UD/Nissan, Deutz (there are liquid cooled Deutzes btw), and Iveco engines are out there. People who say they are not available are LIARS, like the untempleworthy-yes, I worote their stake president, lying is against the Word of Wisdom and unbecoming to the Church in the eyes of Saints and gentiles-Novak, or just too stupid to look in the right places.

There is a four cylinder B Cummins that will fit beautifully, but it is a low revving high torque engine and it will tear up drivelines beautifully.

Your best realistic swap is probably an Isuzu or Nissan. A turbo SD33 or SD42 will fit where the AMC six will, bellhousings can be had, power is not tremendous but adequate-if you have the turbo and the right gears.

The 6.5 GM engine is not my favorite but is reasonably OK when de-electronified and well cared for and plentiful. I would rather have a Isuzu or Nissan.

The VM Cento six used in RPS trucks and in some German Jag saloon swaps looks appealing as well.

Garage I worked for in Hagerstown, MD put a Detroit 4-53 in a two ton truck they used for road service. It had a narrow power band and was LOUD, but otherwise it was serviceable. I think a 3-53 might do well in an FSJ. I once spent some hours replacing the head gasket in a 4-53, and it wasn't too bad to work on. These are two-stroke, so the equivalent c.i.d. is roughly double, in a hand-waving, back-of-the-envelope way. The OP specifically asked for information on Cummins, though.

Earle