This question would not even have to be asked if folks would understand
that it is not necessary to try and stretch the rules for speed in this form of racing.

But, since we can't all put on our big boy pants and play within the rules, well, here we are.

Simply put, a properly done "scale" warbird will look like the full scale plane without squinting to the point of blindness from fifteen feet away.

I propose the use of templates laser cut of 3-view drawings on hard card stock or thin metal, of the most popular planes that seem to get stretched out and all skinnified up.
These templates could be cut with outlines that show incremental increases I wing area that when compared with chord would show obvious deviations in planform.
Likewise for the top and side view of the fuselage.
Simple and accurate.