Hi Fred:
Well, actually you can do whatever you please in modifying the construction of a particular model, but you then run the risk of having fidelity points subtracted. There have been a number of models flown in Classic contests (not just VSC...) that have had a number of structural or dimensional changes, and rarely has anyone objected. It's really a personal integrity thing; if you are comfortable not adhering to the written rules of the event, that's between you and your conscience. For a long time the rules have been "bent" and those transgressions have been largely overlooked in favor of more participation. There have been many cases where tail length (distance between the hinge line of the flaps and the elevators) has been dramatically increased by some in an effort to improve the performance of a given Classic era airplane, and those changes were not even questioned officially. They were, however, noticed by many competitors and the guilty parties were mostly kiddingly admonished. I suppose the old rule of "Let's not hurt each other shall we..." came into play in most instances. I don't think a lot of the competitors in those contests would care to have their airplanes examined closely in respect to plan accuracy...
If this were a true "fidelity to the original model" issue, then virtually all of the models entered in most Classic (and Old Time...) contests would fall short. Hey, long ago it was decided to allow modern engines, modern control system components, modern finishing materials, modern props and spinners, etc. Had these things not been allowed the attendance in the vintage events would have dwindled to just a few very dedicated individuals who endeavor to produce absolute replicas of a given model as it would have been in its actual historic timeframe. But those very gifted and dedicated people are few and far between, and all of them put together would not be enough in numbers to sustain a competition event these days.
So, again, it's a personal thing. Just be aware that if you win a Classic contest with a model that is obviously modified in structure, planform, or even "character" you just might be protested. Likely? Probably not, but just sayin'...
Later - Bob Hunt