confused with lug stud sizes. help

Something doesn't seem right. An 8-3/4 rear axle with only 9" drum brakes? Should be 10 x 1-3/4 if stock.
Are you sure it's not a 7-1/4 rear?

Anyway, I'd check Rock Auto first and see what's listed.
Then looking at the specs for the correct stud, see if there is a longer one available from Dorman with the same spline diameter and thread. Should be 7/16-20 for either rear axle.

You realize that it probably has left hand threads on the driver side? Good idea to switch to right hand threads all around since you want to replace them anyway.