I have a similar sloping clay soil. If you want to plant blue berries, they need acidic soil, so make sure you check the ph of your sloping clay soil. I read somewhere that clay soils are usually alkaline . Before planting blueberries I amended my slopey clay soil with peat moss, just around the blueberry area. I also noticed that my lavenders for bees are doing great on the clay soil and found out that they love alkaline soil. The drainage problem for lavender on clay soil seems to have been solved by the slope.
