WTB 3.55 gears for an 8 1/4

I like the idea of using gears from another vehicle but what is the concern if the ring and pinion are only being used. The axles are not going into the ring and pinion. Just the center section. I am trying to find a grand cherokee with rear disks brakes and hopefully adapt to my 71 dodge. I have heard of this happening before. Will this not happen because of the spline?