I had a blacksmith friend forge the tops flat and basically copy the originals for me. I started with u-bolts like you show, bent at a local shop while I waited.
Not the best picture but you can see them.
https://picasaweb.google.com/lh/photo/rWrnxUcHA5wPyk9snDe_O9MTjNZETYmyPJy0liipFm0?feat=directlink

After I had those made I recall seeing just what I needed on a P-30 or Chevy 3500HD chassis, and thought it'd be worth going to the GM dealer and see what the availability and cost of u-bolts is. I never checked.

Another option is look for or make some blocks that fit the u-bolts like GMC used.
again not the best picture but these cast or forged blocks have curved tops and seats for the u-bolts to nestle into. You may need different yet u-bolts but this slightly curved shape should be easier to get bent for you.
https://picasaweb.google.com/lh/photo/O4Ywa09vkE6UBw3tGMhrYdMTjNZETYmyPJy0liipFm0?feat=directlink

I have also seen similar saddles on the front springs on P-30 chassis, and I have a few loose I can measure. They may not be spaced far enough apart though to use on the rear axle (4" OD tube).

In any case your options are to have yours forged, find the proper ones, or find some blocks to work with/fit easily available u-bolts.

•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-