Changing from 7 1/4" to 8 3/4" rear end questions

92b said
are the driveshaft lengths different between 7 1/4" and 8 3/4"?

Yes, by approximately 1.5". The pinion "nose" of the 8.75 is longer.

92b said
Also is the 7 1/4" u-joint the same as 8 3/4"? Thank you.

Depends which yoke you have on the 8.75. Measure 'em up.
Also be aware that the axle tubes of the 8.75 are larger diameter than the 7.25- that means you will need different shock plates and U bolts, too.