At some point they became pretty interchangeable -- sprint cars and track roadsters. I've seen pictures of Len Sutton's old roadster fitted out to be a sprint car. There might have been some chassis/suspension considerations but, as the track roadsters progressed, they were flat out racing machines, not street machines converted for the track.
A video of Len Sutton in a track roadster versus Allen Heath in a midget. (And, yes John, I can get logged on in spite of my digital ineptness. :-))