I'm into small blocks, not /6, but the plug threads as well as the electrode is longer than anything I've seen before. It actually wouldn't surprise me if the pistons are hitting it.
Nope. The thread length of the ZFR5N is identical to that of a Champion N14Y (original plug on pre-'75 Slant-6s and on approximately seventy-three million LA V8s). The electrodes are longer, which is the point of using these plugs—they move the spark away from metal-wall quenchout zones, for more consistent ignition of even marginal mixtures. Of course, they aren't a universal drop-in for any/every engine, because yeah, you do have to make sure the pistons won't smack the electrodes. There's adequate clearance on stock and near-stock 225s and 318s and 2.2/2.5 nonturbos and a fair good whack of other Mopar motors.
There aren't any spark plugs listed for /6 Jeeps because there aren't any /6 Jeeps, number one. Number two, constraining yourself to what you think you see in the listings sometimes means you aren't going to get where you want to be.