1985 D150 "Mr. Curtis" race truck with a 2015 6.4

Recent news:
I will have to investigate what's going on inside the SRT-10 Dana 60, because when i try to turn one disc, the other turns the same direction.
I kind of expected an open diff... now i want to know if it's a limited slip (good) or something worse (a spool would be really bad).
It also has some play between the pinion and the ring gear, not bad but noticeable.