If you are only getting 3200rpm at WOT, I don't think there a Prop with a low enough Pitch to to even 5000rpm. 1800rpm shy, means you would need an 8" or 9" Pitch,
As mentioned, you need to verify the Tach
My 78 90hp with a 17, turning 4800 was getting 38mph. WOT Range 4500-5000
My 84 115hp...