Well---- First, Welcome to TBN. I have very close to your tractor and I bought a Bush Hog RO72 for use on my mile long gravel driveways and trails I'm going to create on the property. My rear R1's are exactly 80" wide. I'm sure you could use an 84" it would just load up your tractor a little sooner. Along with the roll over box blade I got a hydraulic top cylinder. Believe me, it makes all the difference in the world when using the ROBB. You can make minor adjustments and change the amount the ROBB is digging in - on the go. So rather than my unit digging in and bringing my tractor to a slow stop(oh, boy - it can sure do that) I can make adjustment with the top cylinder and keep going. As you notice - I'm not the least bit concerned that the ROBB is narrower than my rear tires. If I'm doing finish grading - then the setting scrapes very little and is not affected by tire width. If I'm taking a big swipe then its OK too - I will be back and be doing finish grading soon and all will be fine.
I have very nice fertile soil, where I have any. About 25% to 30% of my property is exposed basaltic lava. I call this my moss farm.
I purchased the ROBB because it does three things well - breaks ground with the scarifiers, digs with the blade and pushes with the blade. Box blade can do the exact same thing too.