I have said this before. A wise Flathead guru from a long time ago told me that flatties need heat to run efficiently. The Rprange to stay in he said is between 2000-2800 rpm. Running a 5-speed with a .86 5th gear, 32” tall tires, and 3:54 banjo rear (my avatar) @2500 rpm I am running right around 75 and at 2800 I am at 86 mph (and I have sustained 86 for 2 hours except when I hit a headwind and lose 3 mph due to the weight of all the extra parts I carry “just in case”)! She is one long distance runner with 3 cross country trips to prove it.
All kidding aside, I can tell you my nearly stock flattie does not sound like it’s straining nor does it know the difference one bit at 2000, 2500, or 2800 rpm - she just loves it as do I!
I wish every Flathead owner the same fortune ……