Drive shaft length ???

I just completed the 7.25 to 8.75 Swap in my Scamp, The best advice I could give would be to install the new rear, in your case the 8.25 and measure from center of rear end yoke to center of tranny yoke. Do this with the weight of the car on the rear end, car on the ground or at least have the jack stands under the rear axle housing.

IN my case the amount I needed to shorten was exactly the difference between the 7.25 and 8.75 center of axle to center of yoke = 2.26"
But, that may not always be the case as others will point out.