The biggest problem you'll have with a J200, if my memory serves, is that the motor mounts for the OHC 6 are in a funny position, and the steering column goes essentially through where any other engine would mount. I ran into this problem back in the 70's when I thought to replace the engine in my J200 with the then-alternative Rambler 6 (that quart of oil every 50 miles was getting tiresome). I believe the V8 and Rambler OHV-6 versions had a different steering gear, and perhaps even a different axle, from the OHC models. You may have to do some pretty fancy fabrication to get around this. Gladiators of any kind were very scarce back then, and I couldn't locate the parts, and hadn't the skill to make my own, so I gave up and traded it in on a Scout.
It might be easier now to do this work, since they made them for so many years, so it might be easier to find donor parts. I'd recommend a later front axle if possible, because even if you can make the engine fit, the turning circle of the original Gladiator with closed knuckles was abysmal. If you can make it work, though, you'll have something worth doing. The original Gladiator is a classic design, and to my mind still one of the best looking little trucks ever made.
