OK, serious question here Oldfart. In your wording on engines in the guidelines you've put together so far say this:

"Engine
To be a version of the engine supplied with the car as sold new, or a common conversion. Ie one off engine transplant is not acceptable, but where this was not uncommon would be allowed. Onus of proof as above.
Modifications to the engine".

A "common conversion" for pre-war American coupes in NZ road racing was to fit an OHV V8 into them. I have a 1938 Ford Deluxe coupe that I'm slowly building as a tradtional hot rod, using all period parts, eg, 15" Mercury wheels, cross-ply tyres, 4-wheel drum brakes, etc. If I were to look into finishing the car as a double-threat-coupe, ie, one that could be used on the road and raced, can I fit an OHV V8 engine in place of the flathead, as a "common conversion" if the engine is pre-1960? As stated, the OHV V8s were a common conversion during this period. But, in your wording you've also said "one off engine transplant is not acceptable", so can you clarify?

A 1938 Ford coupe on cross-ply tyres with 4-wheel drum brakes would be bloody slow, even with a more powerful engine, but would be a lot of fun!