My modified/6 could not blow the 7.25 Sure-grip I had. An open 7.25 rear is a bomb waiting to go off and I agree at his hp/tq level he is best to go with an 8.75.
An 8.25 would be ok for now, but if you go with a 360 later on; an 8.75 now is something that can stay with the car as it gets more powerful.
You'll need a different driveshaft and different shock plates to begin your upgrade. Good luck