On my bush-hog, I, and my Dad before me, have always used a chain for the top link to the tractor. (Use a pin where the top link would go, on both the mower and tractor.) That way, if (and when) the back tires of the tractor go down in a ditch or hole, the back end of the mower is free to ride on top. If you use a 'solid' arm, something would get all bent outta shape, with the back of the mower trying to hold the rear of the tractor up.
That said, if you have nothing but a level field to mow, you'd be OK with the top link.