If you go slow, you shouldn't have any problems. You want to make sure not to bog the engine down, so your speed is what makes the biggest difference. You have enough power to run the mower, just not enough to power through really thick stuff. It will also help if you keep the mower up fairly high while cutting. The lower you cut, the thicker it is and the more power it takes.
I run a 6 foot cutter with 30 hp at the pto. You have more power then I do, so the bigger mower should be right at the max of what you can handle. Ideally a 6 foot mower would be better if you where buying, but for free borrowing, you'll be fine.