Start with some NOS shackles and see how they last.

If the rubber in the old axle mounts was worn out and then welded fast the alignment may be off. May be off even if the rubber was OK at time of welding.
Probably worth actually checking rear axle alignment.
Instructions in the shop manual or have an alignment shop check it.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-