If the rear end will be narrowed, you may want to stick with straight-up shocks. Otherwise, 15 degrees seems to be real common, just remember that you lose 7% of the rated load with the angled spring. My '37 fiberglass car has 200lb/in springs in the rear.

With coil-overs, it's common to set the pinion angle parallel to the engine. In my car, I set the engine dead level and did the same with the pinion angle. Tilting the engine more than .5 degree up at the front is about the limit in my car. Any higher and the transmission could not be removed with the engine in place, unless the transmission cross member was rebuilt to lower the tail of the transmission, to create more angle. The actual angle of my driveshaft is just over 1 degree.