I've seen it done, but don't advise it. Mounting the rack to the axle just adds more unsprung weight to it, plus there's the issue of the shaft from the rack to the column. There has to be not only a flex jount in it, but also some sort of way for the shaft to telescope with the changing lengths. IMOP, if you're going to go to a rack, best bet would be to go ahead and do a complet suspnsion swap to a suspension designed to work with a rack and pinion steering system.
I got a question about your current setup. You say it's welded to the frame? Was this done by a PO? The box shouldn'e have beeen welded to the frame, very unsafe, plus it really impedes any service to the box itself.
