cross section of the fuselage means little
unless you completely blank out the prop -like scale models of GeeBee R1 craft
The closer a large dia. flat plate is located immediately aft the twirling prop, the lower the thrust will be .
You can do all kinds of cowl shapes on our small models and never make 2 cents worth of difference in performance.
for max efficiency you will extend a spinner and prop as far as possible ,from the larger cross sections of th fuselage
sorta like the spinner /cowl on turbo props.
here is an example