As far as I know the NovaRossis are "re-purposed" car engines. They might work out OK in a speed plane, but I can not say. They will most likely [because of lost torque due to high timing] be harder to find suitable props for. Just my guess.
The 6.5 x 5 prop is the best one that I've used for the CVA .15.
I was hoping to see a 6 x 6 alternative if you were to build a twin or tri-motor.
APCs CAN be reworked [shortened] to help match the load to your engine, but it is of course more work that takes time away from flying.
You could try reworking the 6.5 x 6 just to see if trimming it down to 6 inches or less allows the engine to hit resonance. The 6 pitch in the mid 20K range ought to give impressive speed.
There is an invisible line that you can cross while reworking a prop. The engine will seem lifeless, unresponsive, growing hot, if there is just slightly too much prop, so recognize that early and don't make it suffer, just kill it. Take the back of an Xacto blade and scrape some meat off the prop blades' airfoils, give it another try and sometimes MAGIC happens.
There is another approach, but it still requires work because the props come semi-finished. Steve Wilk at Eliminator Props has an almost infinite selection of real carbon strand, hand laid speed props and props in this size range are probably $10 - $12 each. With a hot twin, you could roast 2 glow plugs, break 2 props and be out over $30 just for "consumables"........!
This would test your love for this sport.