Those numbers are based on RAD (the people who make the snow blower for Kubota). I think for a 72" wide front mount blower they recommend more than 35 pto hp. Personally, I say BS. I have a 72" front blower on my 4240 (36.5 pto HP) and HP is not even close to being an issue. Normally I run the engine at about 2000rpm and even then with the reduced hp I don't have a problem.
On really deep snowfalls I open it up and run at full pto speed. But it's not because of a lack of HP but I do it so the impeller can throw snow as quick as possible. The biggest problem with a snow blower on the big snow falls is the restriction on the amount of snow that has to go into the smaller round impeller area.
If hp is ever an issue you can always go slower. It's not really going to cost you time because your removing the same amount of snow as a smaller blower moving at a faster speed. On your second pass there will just be less to remove. My advice, I'd get the 72" blower.