You definitely need some defection in the U-joints, when I mentioned all of the transmission and pinion shafts should be in alignment, it was meant that they were all sharing the same slope as each other. They will have be at different heights, and can be to some extent, a bit side to side to side from each other, this is where the driveline angle comes from(if there are both up and down and side to side differences the actual angle of the driveline is a little more fun to measure). You will find the engine is slightly sloped down to the rear, and your pinion shaft at the differential is pointing slightly up towards the front. Depending on your current driveshaft, and if it has an even number of U-joints your pinion shaft should be pointed up at the same angle as your engine is pointed down. If you have an odd number of U-joints, this angle might not be the same and you may have to shim your axle to rotate it when you install the brownie and end up with an even number of U-joints. To end up with smooth vibration free rotation of the driveline, the angles between the transmission and driveline, and driveline and whatever it connects to have to be equal and opposite so that they cancel each other out, when there are two U-joints in the system.
I have a single speed rear, 6.17 ratio.
