I acquired a 60" single auger rear blower when I upgraded to the Cab model. I have the Rear R3 Turf's in the wide position and don't notice a "coverage" problem. Will a 3520 handle a 72" - of course - BUT - how deep is the snow? How wet is the snow? How fast do you plan to travel? How heavy is the blower? Will you need front ballast / chains? Is your area flat / hilly?
For 98% of the time - in my area - I'm sure my 3320 would handle a 72" blower, at other times, with a large wet drift or clearing the pile left by the rural snow plow in the driveway, I'm sure a 60" could bog down even your 3520.
Without being well versed regarding your local snow conditions and how you plan to use the equipment, I don't think any of us can give a definitive answer.
I have a 500' drive which is flat in central Wi. I'm looking at a 72" from Northern Tool and it says you should have a 40hp tractor. Not running any races ,just want to clear thne snow.. got R4 tires on it which makes it stand 68" wide at the rear.