Generally I believe that 5.7 and 6 inch rods will require a piston with the skirts grooved. Because of the longer stroke of the engine you need special pistons to fit in the 4.03 bore and clear the rods.
If you use the 400 rods from what I understood that would be the only way to not have to clear the the piston. That was the "old school" way of doing it.
Your stock 400 crank would have to be resized as well since the main bearing size was larger. ( just like mortec said)