7 1/4 to 8 3/4 Newb Questions

RRR is right, you are going to have to shorten the drive shaft.
7 1/4 10.09 inches from center of ring gear to end of yoke
8 3/4 12.35 inches, that is a 2.26 inches difference in length.
So measure the drive shaft that is in the car now and either take 2 to 2 1/4 inches off, with 3/4 to an inch of slip yoke travel or find one that is that much shorter.
Other Mopar axles for reference,
9 3/4 Dana 13.47 inches
8 1/4 and 9 1/4 are both 11.69 inches
These measurements are for Mopar factory yokes if using aftermarket yokes measurements may very.