That angle the leaf spring is at (as seen from the side) effectively steers the vehicle from the rear when it leans in a corner, the same direction the front wheels are steering only less so, causing something called roll understeer which enhances stability at speed. (If it were opposite, the rear end might wind up whipping back-and-forth down the highway in a condition similar to a skateboard with the wobbles.) Kinda helps with packaging also, keeping the floor low under the rear seat. As far as how much angle exactly; "some" is about right, you can measure any leaf-spring-equipped car or light truck built from the sixties-on and copy what they used. Once doing that, then just aim the diff at the trans output (at ride height of-course, and knowing that the engine crank centerline and the rear end pinion centerline need to be parallel but not exactly lined up so the u-joint roller bearings roll around a little and spread wear so it's not just in one spot). On another note you did good by having the leafs spread as wide apart as practical.