A lot of Massey Ferguson 4xxx series tractors (4900, 4880, etc) came with a Cummins VT903 set at 320hp +/-. As the 903's puked out, a lot of farmers repowered with various other engines,mostly in the 14 L size. Big cam Cummins and 3406 Cats were popular re-fits with a lot of these repowers also boosting the HP of the tractor.
This has been discussed a bit over on newagtalk forum a few times and a guy that goes by "onion farmer" from southeast Washington says he owned a KTA 1150 powered MF 4900 with 12,000 hours that puked a trans. Could be the same tractor you saw Truck Stop , or not. He doesn't list what town he's from in Washington, just says Southeast area. It would certainly be a unique tractor with that big Powerplant stuffed in it.
https://talk.newagtalk.com/forums/thread-view.asp?tid=380460&DisplayType=flat&setCookie=1