I think Fox was the first to tilt their engines in 1969, Rupp started doing it, along with a few others, the following year. Someone told me that they had a conversation with Mickey Rupp about it, and he said that Rupp started tilting their engines to compete with two stroke dirt and trail bikes. The more it looked like a motorcycle, the better it sold. I can't confirm this statement, but it makes sense.
I don't think there really are any performance advantages for slanting the engine. If the advantage was for hill climbing, I can't see the point of a slanted intake. In fact, the first year that Rupp slanted the engines, they used a diaphragm carb (and they were finicky) at best, and the performance was probably hindered. They switched back to the bowl carb with the use of a slanted intake in 1971.