Fixing the frame isn't really all that difficult. Take out the crossmember and grind it, then make up plates (2) for both sides of the rotted area, that extend past the rot to good metal... Cut some oval holes in the plates and weld along the plates on the top and the bottom, and weld inside the oval areas you cut. Do not weld vertically along the ends of your patch plates (also called "Cheek" plates)as you will end up with stress fractures along those welds... Then, as above, put the crossmember back in with tight-fitting bolts, and I would even use grade eight bolts... They are cheap enough insurance!