Just to throw this in on the crossmembers, if your going to spend the time and money to box the frame, I would customize the crossmembers to fit the entirely boxed frame. But again, without throwing some serious torque at it, you shouldnt need to box it.
Your concerns here are the engine torqing over when being romped on, and/or having, say, a 4 link suspension under it thats gonna put stress on the frame. You still havent told us what your planning on doing with the truck.
I wouldnt box it short of 400+ HP and taking it to the drags... Just my opinion.
