Looking for 8 1/4 shock plates or substitute

The shock plates are only specific to the size of the axle tube so any axle with 3" tubes will use the same shock plate.

Or, you can just elongate the holes on the 7.25" axle and use them. I have been using the shock plates that were on the car with the 7.25 for 50k miles now to clamp my 8.25 to the springs.