We are not talking about race cars or two shaft. Geometry tells us that if two parallel lines are intersected by a transversal, it creates congruent angles. That means the Ujoints will be operating at the same angle. If the two are not parallel, they will not. The angles should be equal and opposite for a normal truck with leaf springs. There can be some tolerance here, as in: 1/2 deg. Ujoints are designed for 1-3 deg angle.
The differential windup will tend to straighten out the working angle, that's OK. Zero angle is not recommended because this does not "cycle" the ujoint enough to lube the needle bearings.
A motor should indeed be set up with the carb mounting at level. Not close, but level. You don't screw around and set the "float level" and "float drop" precisely and then set carb at an angle. It is known that most Chevy motors have a tilted intake/carb mounting surface relative to crank shaft centerline. Set carb mounting level and the tailshaft will be tilted down 3 or 4 deg(probably 4).
This subject always creates big disagreements because people start talking about drag racing, circle track, wind up, suspension type and the many website discussions. I am sticking with the basics.
It's not rocket science but it is important to avoid ujoint failure, vibration and fuel delivery problems. With several "angles" to set up, close is not good enough. Errors will compound. My first boss would always ask "If it's not right, what is it?" I would have to answer.... "wrong".
BTW: Motors do not sit nose down in my experience. Joe's first post has great info, his second post is what I'm discussing. Where is the first Joe?
