Pain in the asp, on my Nova. But the problems I had there are not likely to be something that affects a GMT400. (Mostly, but not entirely because the friggin' aftermarket spring pair I bought were made incorrectly.)
Buy new U-bolts. Consider replacing the other spring and it's U-bolts at the same time. That spring has been down the same roads as the broken one. That way you don't have mis-matched rear springs--one new, one old. If you're installing "used" springs from a Treasure Yard, consider fresh bushings at each end.
U-bolt ends from an axle-swap on my '88 K1500 compared to new replacement:
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