A great resource for online Chevy manuals is here
http://chevy.oldcarmanualproject.com/

Here's the 48-51 truck shop manual
http://chevy.oldcarmanualproject.com/shop/1948_51truck/index.htm

These pages list numbers of springs and length
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0316.htm
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0464.htm

For more detailed info try http://www.eatonsprings.com/ they say the have all the original specs, but don't expect them to tell you and then have them made by someone else...
They made a set of springs for my truck and did an excellent job, and very reasonable price I thought, and they had all the pins, bushings, and related parts I needed.

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-