I think the only real solution here is to sell the truck ;-P If the hole is centered in the frame (height) it will have the least effect on strength. I would weld a section of tube inside the hole to give it a little more strength. I would not box it however. If the plate used to box the frame was square, you would have stress concentrations at the corners which could lead to cracking. especially on an old C channel truck frame with riveted in cross members. The frame has some give in it, and a stiffer than the rest section could crack as the whole thing twists. As far as above the axle not having the most stress... the axle is supported by the leaf springs. The shear force should be greatest at the spring mounts, the bending moment will be greatest in the middle of the span, or above the rear axle. This is why you will see some I beams with a web that gets thicker as it approaches the middle of the span. Pushing the flanges further apart greatly increases the resistance to a bending moment. It has no effect on the shear strength tho, so having a tall I beam at the ends of the span is a waste of steel. For this reason, it would be safer to drill a hole than to notch. Notching effectively brings the flanges closer together which exponentially reduces the resistance to a bending moment where it is most needed. I know it has been done, but plenty of people have built cars with no front brakes. Doesn't mean its a good idea.