Using an axle with more drop is one way to lower the front of an I-beam axle truck.
I've used a later model axle under my truck that has 2" more drop than the stock one, and I've made up for it with taller springs.
If you want the drop just use the original springs and do all the work and effort to swap to the later axle.
http://www.stovebolt.com/techtips/bigbolts/p30swap/

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-