I run a 72" flail with "V" knives with 25 PTO HP. I have no problems either with what I cut or speed but I cut only what the mower is designed for.
It's rated for "up to 1" material.
I have .34 hp per inch.
You will have .36 hp per inch.
I think you will be ok.
Like islandtractor said, just be reasonable in what you cut