A 6.9 put out 286 hp and 405 foot pounds of torque. These pop up for sale now and then and would be a direct bolt in replacement.

They are also limited-slip.

I've done a few of these. It is really not hard if you have a simple hydraulic jack to lift up the differential to steady it while you bolt it in.

I don't know if a 126 differential would work. Perhaps someone with an interchange book can tell you?