Here's a couple video's that show the importance of having the trans and pinion centerlines parallel. Pretty simple concept, really. The only trick is to determine where the pinion angle needs to start at so the 2 angles are PARALLEL UNDER POWER. But this should put to rest any of the nonsense that has already been beat to death about the angles not needing to cancel each other out - but I doubt if it will. Enjoy the videos.
https://www.youtube.com/watch?v=gmV4qwLfOMYhttps://www.youtube.com/watch?v=T4P75ZQvpws These are excellent videos. I have no argument with either one. However, neither addresses the issue which is, what happens when the tailshaft is 3 degrees down and the rear u-joint is higher than the front and negative pinion angle HAS to be maintained.