I ran Cox Conquests in F2D for a long time. Without going all high-tech the prop procedure was to start with a 7/4 that would stay in one piece and trim it down until the engines goes the RPM you want. It seemed like 23,000 was the breaking point for a Conquest. Of course that was for 4-minute runs, not 40 seconds.
Back when we could run unlimited nitro in F2D the Nelsons beat the the Cox because they would tolerate a lot more nitro. But in Perky with 10% nitro and only needing run 40 seconds, a Cox might be a little closer to competitive.
40 seconds is 90 MPH.