I've run a 5' cutter on a JD 955 (about 30hp CUT) and an old M-F compact tractor (side mounted type and probably about 20 hp). Your machine will probably spin the blade as long as you don't ask it to cut anything too difficult, but there will be a pretty big pucker factor when things become less than perfect. Both of those tractors had front weights and would still lift the front end off of the ground if you raised the 3ph too quickly! I would imagine that pto hp will have less to do with your final decision than your tractor's ability to counter-ballance the mower.