The BB720X would be a good choice.
I use a BB720X with a MF 533. The 533 is the 2007 modernized version of the 135 with a NA 202 CI Perkins engine rated at 45 PTO HP.
With 4 front weights, the 533 handles the 720X well but I sometimes find myself in low/2nd in the very heavy stuff.
I would not go to 7' unless it is light and the cover that you plan to cut is also light.
I also have a 7' BB840X that I use with a 60 PTO HP FWA MF 2635 (220 CI turbo engine). I've found myself in low/3rd in heavy stuff before and can imagine situations where the mower would not clear well requiring low/2nd.