8.25 or 8.75 rear

If it were me, I'd put the money into an 8 3/4. It's a better rear end all the way around. The 8 1/4 is cheaper but that's it's only advantage except for being a few pounds lighter. Other than strength, another nice feature of an 8 3/4 is that you can keep several 3rd members on hand and swap them out in less than an hour. That makes it nice to change from street gears to strip gears and back again. To change gears on an 8 1/4 means resetting everything up in the rear end housing.