Let me know how that turns out. I believe that will work fine for a cutoff hardie, I am planning to do the same, with one exception, to one of my extra/old ones. My plan is to forge the head or hammer end to fit the hardie hole so it will be one piece and a little shorter. Wedges are too tall as is, so a little shorter is great. Welding on a shank will work fine too, all of my hardy tools are homemade, most of them welded shanks that were cut and forged from other stock, some are one piece forgings formerly axles or solid driveshafts.
Definitely reharden them, but not too hard, softer is better , it will deform eventually but can be reforged, but if it chips , that metal is gone/lost. I do a flyby night hardening on mine, I work them until done, then just let it cool until just barely red and quench.
So anyway...