The choice of the mower size depends on a lot of factors. My tractor is much smaller than yours, but I went with a 60 inch mower and found that at the speeds that I can safely and comfortably use around here, I could have used a 72" even though my 19 pto horsepower is on the light side for it. But then the same unevenness of ground that keeps my speed down might make the larger mower yield less smooth results, dunno.
I would go with the 84" unless you can really scoot over your property, and live with the cut quality, also taking into consideration such factors as space between trees and rocks, acreage, etc.