There is no " right " angle, each car or truck can be different. The correct angle is what ever creates a 3 degree or less u-joint working angle at the transmission tail shaft and rear axle pinion yoke.
Measure crankshaft / transmission angle, then measure the driveshaft tube angle, the difference should be 3 degrees or less, but not straight. The angle can be sideways or up and down, or a combination of both. Next measure the driveshaft tube then the pinion angle, it should be the same as the front or very close to it, again, less then three, but not straight.
The u-joint need some angle for the rollers inside each cap to work. Three is a good easy number and not hard to get. You may or may not know this, but u-joints speed up and slow down the rotation of a shaft as they turn. As long as both angles are to same, the speed difference is canceled out and you get a smooth rotation. Get one off more then the other, and you can get harmonic vibration.
Before doing any welding, the total weight of the truck should be on the frame, or the frame should be at drive height. The pinion angle will change as you load the springs down. This also true for the front springs, the frame needs to be setting at finished level. You can do as I did and use straps to pull the frame down to the axle, I was just estimating, but got pretty close to final height.
Get yourself one of those cheap angle finders, I bought mine from Speedway Motors,
http://www.speedwaymotors.com/DECO-Angle-Finder-Tool-414-Inch-Diameter,2916.html?utm_source=bingcse&utm_medium=cpc&UTM_Campaign=CSEBing&CAWELAID=530009170000024473
They work really good for this job.
Joe
