Let's be clear: Small Block Supers are not supermodifieds. Never have been. The name is marketing hype. That's all. They are a weekly class unique to Oswego. On the other hand, from a fans' perspective, 350 Supermodifieds are supermodifeds but with smaller engines (and a few other differences).
350 Supers are more affordable and a class where an older supermodified chassis can be utilized. Sort of similar to the relationship between Tour Modifieds and Sportsman/SK Modifieds. On dirt, you have nearly the equivalent with Big Blocks, 358's, and Sportsman.