Below is the 1965 AMA engine angle spec for 1965 Tempest and Lemans as I already pointed out. Not sure where the "no more than 3 degrees down" comes from (believe that is for Chevelle, but seems the factory did not have an issue with more than 4 degrees down and no vibrations.
I don't think the engine down angle should have a lot of bearing per say, rather it should be the balancing out of the two angles, engine & pinion. How do those up in the sky 4-wheelers do it with their radical driveshaft angles?
If your crank centerline and pinion angles are equal but opposite each other you shouldn't have driveline vibration - in theory. However, the minute you accelerate, the pinion will raise up and the angle up will increase and you may experience vibration.
So get the angles to cancel each other (neutral), then drop the pinion angle down 2 degrees and take it for a drive. If you have any vibration, then try 1 degree down from neutral. Does it improve or get worse. Make these changes from your base line of the angles cancelling each other out and simply test drive it. Measurements must be done at ride height/suspension loaded.
I found this explanation of the engine/pinion angles and you also want to include the driveshaft angle itself. So it can be more than just engine/pinion angles cancelling each other out. Maybe this will help.
The yokes at the trans come in different lengths. I read that one Chevelle guy switched the long yoke length for the shorter yoke length and that improved the vibration if you experience anything- but this would change your driveshaft length.
So just set the angles to cancel each other out, drop the pinion angle down 2 degrees, and test drive. Then adjust as necessary and road test. Each car will be different and pinion angles can be different for different driveline combinations, so 1 size may not fit all.
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