adjustable is just an easier way to bolt them on. gives you some leeway when you dont tighten the u-bolts down equally. in his case, i would loosely bolt the u-bolt plate up so its close to square with the spring plate on the axle, bolt bar to u-bolt plate, put frame mount on the bar, rotate up to the frame, mark holes and drill. then measure the hole locations just drilled from repetable marks on each side of the truck, mark holes on the other side, drill, then bolt it all together tightly.
You dont want preload in the bars. it changes pinion angle and makes the ride worse than it will already become adding the bars. some guys twist them up at the track stiffening the rear spring rate at the cost of a worse pinion angle. a spring clamp would be better in that situation over twisting up the bars or adding adjustable shocks.