My 70 ton crane is around 95,000lbs with a 6v92, and while its geared pretty low (6.53), aren't you wanting this for pulling your big yarder? I'd hate to add another 40-50,000 to my crane and haul that around with the 6-92.
If you're taking the time to shorten it up, you just as well do what you want power wise while its down. Otherwise you'll get to using it and won't ever get to replacing it. I've got a big cam cummins sitting on my shop floor, that I was going to repower a crane with 8-9 years ago.
Air to air B model cat? Add a air to air, motor mounts and some minor wiring and gauges. If you found a KW donor you could swap mounts and everything.