I'm working on a new plane meant for speed (100mph+ is the initial target.) I want as clean of an airframe as possible without adding to much weigh or complexity. The plan already includes a gentle sweep from spinner to tail. Would drag or any other aspects that affect flight at high speeds be affected if the fuse cross section were square instead of rounded at the corners or fully circular? How much would it help to fully cowl the engine (not so much as to limit induction or cooling?)

Thanks for any ideas on this.