If you're talking about reinforcing the area where you want to drill holes off center (or maybe you already drilled) (

) a 1/4", 4 x 4 steel plate, welded all four sides and centered on the hole, should do the trick.
If your talking overall reinforcement, it's pretty complicated and might not be beneficial. The Mac trailer, is, by design, quite flexible. This has the effect of distributing the loads more evenly over both the trailer and the boat and reducing the peak loading.
Doing the actual numbers would be difficult and fairly time consuming, and ultimately I believe, of little value. My guess is that, in general, the most highly stressed areas are between the front bunk and the rear spring hanger, and the single highest stress point is right where the front spring hanger is welded on. You could reinforce this area by "boxing" the rails along the length, and it certainly will be stronger in that area, but you also will have removed most of the designed in flexibility and the peak loading elsewhere on the trailer might actually be higher. The peak loads on the boat will almost certainly be higher.