If you lower the truck by removing springs it gets lower in two ways. One by lowering the frame closer approx 1/4" per spring removed (spring over axle setup only). And by letting the remaining springs sag while they try to carry the weight of the truck.
This approach reduces the load capacity of the truck.
If you take out to many the axles will be close to the bump stops on the frame and could be a pretty poor ride when they bottom out.
As you suggested, you may need new shorter U-bolts. It is also strongly recommended that u-bolts be discarded and replaced whenever they are removed.

Why do you need to lower it? Or what are your plans for using the truck?

Grigg

•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-