I am using the original axle on my 38 GMC running a buggy spring with radius rods and have not welded anything to my axle.
As an engineer I have thought about welding to the axle many, many times and decided that although it is possible it may not be truly safe. I reason, as an engineer, that any weld transverse to the direction of any beam, tube, axle etc. is detrimental to the original part.
A weld longitudinal to the part is typically okay. You will notice that buildings, machines etc. do not have welds transverse to the long axis, by transverse I mean across.
So how did I do it? For my current design, I spent a little time fabricating a bracket that bolts on in place of the original leaf spring set up. To do this and run radius arms I had to make the bracket long to the outside of the axle so that the radius rods would angle outward. I then welded/fabricated another bracket that welded onto the bolt on bracket to which, the radius arms were welded.
It does not look as clean as a welded on bracket or a new axle because of the bolts but as part of my design I wanted to run as much of the original vehicle as practical. Of course the bolts could be cleaned up using a fabricated cover that would be keeping to the theme of the vehicle, say a skull, spade, whatever.
I may not stay with this design but it will work and be plenty strong without worrying about the end of my axle breaking off under load as I like to drive my cars everywhere and for long distances.
I will take a few close ups of this set up and try to post them soon. Of course with my track record of pic posting it may be awhile. I would rather work on my truck than sit at the computer. lol