The 12 hp (gas engine, not diesel) Gravely that I had could be gotten with a 50" finish mower on the front PTO. I used it with the 30" brush hog; never really bogged it down.
I'd say the 72" finish mower would work fine unless you try mowing fast in high grass. I can mow tractor-hood-high stuff with my 18.5 hp 4010 with its 4' bush hog at max speed in low range. Think it's PTO hp is about 16.
Ralph