To go from a 7.25" rear to an 8 1/4" you need to shorten the driveshaft about 1.6". Obviously the way to do that would be to measure your car, as they're all a little different. But the difference in the yoke positions is ~1.6".
The brake cables from the 8 1/4 probably won't work, as buckshot mentioned there's two different styles, and the early and late cars used different styles. So more than likely the 8 1/4 has a different set up than your '68 7.25. But the cables on your car should work for the 8 1/4. You'll have to loosen up your e-brake tension adjuster all the way though, because an A-body 7.25" is not exactly the same width as an A-body 8 1/4". A-body 7.25" and A body 8 3/4" rears are about 1" different in width, with the 8 3/4 being wider. Not sure how the 8 1/4's compare, but it should be within the range of adjustability of your e-brake cable.
