I just did this a few short months ago. My frame rail was hammered in from hitting a curb. I originally expected to have to replace the whole rail piece with one from Moss but ended up making my own and rehabilitating what I had. The trick was to drill out the spot welds on the bottom and peel up that piece which gives you access to the rest. You can cut the welds of the front of the box and remove that too. Like most of the car it's just origami sheet metal, so making replacement pieces from18 gauge is not difficult. Then just get everything aligned between the two rail ends (I had one good side) and with the same upward bend. I MIG'd most of it into place, maybe a little TIG too where there was brazing so I had to use silicon bronze which plays nice with that.. Measure, measure, measure, align, the usual drill. End result is on the last photos.
BTW, the Moss replacement piece doesn't look close to the original, it's pretty cheesy IMO, different fixtures, etc..