That 12valve has a weight of close to 1300# give or take, with all the accessories on it. The frame,and driveline won't handle that weight, or the torque. And I'd bet it's too long (I6) to fit under the hood without severly chopping the front of the tub out. The reason they went to the longer nose in '71 (?) is to put the Jeep I6 engine in. And the Cummins is quite a bit bigger yet. Just my opinion, either do a 4BT, or a v8 swap. Easier, and less expensive when you start to factor the driveline/frame requirements.
'96 XJ 6.5" lift, 33's, HP30, 8.8", 4.56's, 4wheel disc, spooled rear, locked front, SYE, skids, yada yada yada.'51 Willys CJ3A 32's, 5.38's, locked front & rear, small block, Muncie 4speed.'76 CJ5 - UNDER CONSTRUCTION - resto/mod