Jay,
Adding larger holes along the side of the beam box, i.e lightening holes, is a huge no-no. The shear loads are carried through the side of the box and you are compromising the structures ability to handle the compression loads applied from the bolts when the beam is mated to the hull.
I will add one caveat; outboard of the beam bolts, i.e where the dolphin striker attaches, you may be able to reduce the shear capacity some. I would not risk it on my boat, especially seeing as the original box has already corroded a fair amount.