Probably best to get close to where the seal was running previously, should be about the middle of the travel in the slip portion. As you stated, this would be best done with the truck at finished weight.

For some other very helpful driveshaft info check here:
http://www.stovebolt.com/techtips/driveline_setup.html

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-