The difference in length from the axcle center line to the center line on the u-joint is about 3/4" with the 8.75 being longer. If you have 1.5" or more of extra depth available on the slip yoke you can do it with out cutting but if you have less than 1.5" you run the risk of the yoke bottoming in the tranny.

BTW, how did you wind up with an 8.25 in a 70 Satellite? The 8.25 is a stout rear end, not as stout as an 8.75, but certainly enough to live behind a mild big block or a hot small block.