You should find a lot of your driveshaft questions are answered here in the Spicer driveshaft manuals
http://www.stovebolt.com/techtips/driveline/setup/index.html

If you're not using a factory driveshaft (can't understand what you're doing) then call a good driveline shop for both the expert advice and the new shaft.

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-