Well, if you can scrounge a 48 and later (not sure how late) GM 1.5 or 2 ton rear axle with round tubes they are likely 4" and likely for narrow springs like you need, you could use those spring perches just measure to be sure.

•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-