This Tech Tip will lead you to all the information you'll ever need to know about driveshafts and their angles.
http://www.stovebolt.com/techtips/driveline/setup/index.html

The short answer is put the engine in any how you want to so long as it's straight in the frame front to back, not sitting sideways. Then match your pinion angle to your transmission output shaft angle.

So long as the engine oils its self properly it makes no difference what angle the engine sit at in relation to the ground or the truck frame (within reason). They key is matching the angle of the rear axle pinion to the engine/transmission.

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-