No expert, but I think butt welding it would just anneal (soften) the spring steel rod at the joint. It may eventually just bend (twist) at the join. Or it might be fine.
A better way might be to make some kind of tube sleeve with 4 welded on nuts. The rod ends could slide into it and be pinched by bolts tightened through those nuts.
Regardless, if welding you might want to use a short section of L channel across the rod to back and support the weld joint.
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65 Laurentian post, 67 Grande Parisienne 4 door HT.
ok. Its been a while but rod is now fixed and in place and working properly. I took the two pieces to a blacksmith. Heated both to take temper out of metal. Then welded and re-tempered properly. Install was a pain but with 2 sets of hands quick and easy. Friend of a friend was the blacksmith so NO CHARGE! The end.