I opt you go square! It's cheap and very strong! Also if your running duals you can build it with around 18" of travel like Mini said and if something happens to your rear driveshaft, it will extend long enough to be used as a rear driveshaft and still have pleanty of travel. But even if you don't decide to go square, I would build one just for a spare because like I said it can be used as a front or rear!!
