Usually you have a spring pad on the axle, then u-bolts clamp the springs to the axle and tight to this seat. Here is a poor picture, but may give you the idea: http://rides.webshots.com/photo/2651856980080251109LSnfpj
Welding the spring seat to the axle tubes is where you can adjust the pinion angle.
Or if the spring pads are already in the right position center to center, but the wrong angle, and you don't want to move them you can make a shim like this to correct for the pinion angle.
http://rides.webshots.com/photo/1484200068080251109FlXuOW

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-