8.25 or 8.75 rear

8 1/4 would in most cases be cheaper to find. it will be large bolt pattern too. should hold up fine for what you want. may be harder and more expensive to find one with a sure grip and decent non highway gears.

i prefer the 8 3/4 myself. easy gear swaps and plenty strong . they are small bolt and will cost ya to convert to large bolt pattern. gears and sure grips are easier to find . gonna pay though for them though..