Good news and bad news.
IN A PINCH you can elongate the holes in your shock plates so they will accept 3" U bolts, but you may need to do a bit of grinding to the plate to allow the nuts to seat in their new positions.
You DO know you'll need a new driveshaft (or have your current one shortened)? The nose of the pinion is further forward on an 8.75 than it is on a 7.25 rear. And depending on the carrier and yoke you have, it may require a different (or adapter) U joint.