I used 6000 rpm static to guestimate the prop size. At these rpm, about 20 lbs thrust is generated by the Zenoah, which would be very fine for that warbird. For 3D I would have used a more free-spinning setup like large, low-pitch prop at 7500 rpm
Gas engines are not lugged down at 6000 rpm, but operate in their best torque range, depending on muffler system used; In addition, the low rpm growl of the gas engine suits a war bird much better. In a dive, the engine will spin up, and the prop may even produce sonic slap, but only if the pilot wants it to.
Maybe an example will illustrate my point of view, and experience which led to it:

As my first gas engine, I used to fly a Quadra for towing. Set up around 8000 rpm, it was very disappointing, and would hardly fly the tow plane. Once propped for 6000 rpm, it was ready to go and do it's thing.
Over the past two decades, rpm at our flying field continued to drop in order to meet noise limits. Amazingly, performance was not impaired, and remained at the same level, or even improved. That could be judged, when an old-style pilot reappeared with his screaming plane. He was not able to mingle with the new generation anymore, and did not pass noise measurements, so the guy was in double trouble.
Conclusion: RPM by itself are a dream, not nesessarily leading to real world performance.