description of your mowing conditions would be useful
3ph or drag type? 3ph, no out of the question. 8' drag type? some will say yes, but personally would not invest in a mower of that cost for 40+ hp tractor. overkill on mower size. are you able to test that setup in heavy grass before buying?
my choice would be 6'. 7' is generally considered a bastard size.
a drag type finish mower might work but won't last long in rough conditions, plus your cut height will be limited
my 8' twin spindle 3ph mower (2000#) is well suited to my current tractor size. had a 6' 3ph HD mower that was well suited to my previous mx5000.
imho, good luck