[dsraven]

I have done something similar on a 57 frame in the past. there is still 3" of frame left above the notch. thinking back, if I was going to do it again, I would lop off the frame ahead of the axle and fab a whole new rear section. boxing the frame ahead of the notch as well. actually, if I were to do it again, I would do what you have done and start over with new rectangular tubing. most guys cut cross members and fab new stuff here and there anyway so, why not. I have often thought, while checking some builds, that by the time guys cut the front end off and weld in a different front clip from some other old car, then cut the rear off to install an aftermarket or home made c notch, then boxed the rest of the frame, the only part of the frame they actually re-used is the center section under the cab. they might as well have spent the time on building a frame jig and built their own. that way the old car front clip could be properly installed to a frame made the correct width and height to match the "new" clip and the rear section could be made to match the suspension and frame width required back there.
just my thoughts though.