7 1/4" to 8 3/4" rear end swap advice

When I had my replacement driveshaft cut down I measured with the slip yoke pulled out the same distance as when the 7 1/4 was in. And I got a dimension about 3/4" shorter than the parts book called for. So when i specified a length I split the difference between parts book and my measurements

The trans originally had a rear seal that was flat, it did not have the extension bellows. But the replacement seal has the bellows.

The resulting length makes the bellows ride on the part of the slip yoke that starts to sweep up. Had I gone with my measurement it would have been on the flat part of the slip yoke.

I forgot to mention that the OEM stock driveshaft was not the length the parts book said either