I'd look around for the biggest pull behind rotary cutter your Kubota can handle. I'm suggesting pull behind because the HP requirements are less (not much less though) then a 3PH type cutter.
Your B3030 has about 24 PTO HP. You can easily use a 60" cutter, but might get away with an 84" pull behind if your acreage isn't too overgrown and you mow often enough. An 84" cutter would be pushing your tractor hard, I'd think.
Land Pride recommends 20-50 PTO HP for their RCR1872 72" 3PH cutter. They recommend 30 PTO HP for their 96" pull type cutter. Extrapolation would result in about 25 PTO HP requirements for an 84" pull type cutter, however Land Pride doesn't make an 84" unit.
Look at Befco, Howse, King Kutter, Servis-Rhino, Bush Hog and others.