diff pad set up
I just went through this myself... Don't get crazy with the what if's; stick the angle below and call it.
The cars doesn't need to be uber level, this is a
dynamic relationship from a fixed point, it could be upside down and would come out the same. If your still worried, adjust the ride height how you want it in the front, add the everyday load you would have in the rear, and do the damn thing.
From this article:
http://www.carcraft.com/howto/91758/
"According to Currie, the average car crafter should strive for between 1 and 3 degrees between the tailshaft of the transmission and driveshaft, and 1 to 3 degrees between the driveshaft and pinion. Furthermore, the two angles should be nearly equal (between 1 and 3 degrees), but always opposite (see crude diagram)."
Here is a picture:
http://www.carcraft.com/howto/91758/photo_08.html