in order to swap the pistons, you'll have to pull the motor, put it on a stand, pull off the head and the oil pan and extraneous junk underneath, remove rod/pisons, possibly machine stock rods to accept aftermarket pistons, put in rod/pistons, put the whole motor back together and put it back in the car. Its far from a small job. If you're set on doing an n/a buildup, I'd replace the rods while I was in there with something lighter, and use better rod bolts to accomodate a higher redline.

:edit: or more pertinently, are you allowed to swap?