We bought a used
L3130, I think a similar frame size to the
L3700SU, but less hp. It has R4 tires and the way it came it is 72" wide at the back tires. This is the process that I went through,
I looked at 3 options,
1) flail mower. I was not sure my tractor could handle the 72" one, so I would have to offset to cover at least one wheel track. Safest choice around people and animals as it does not send rocks flying like bullets.
2) rotary cutter/mower. guideline is 1 ft per 5hp. I have 24-25 pto hp, so really a 5ft would make sense, again the tractor width vs the implement. It is a lot of weight hanging out back there.
3) rear finish mower. It sets a bit closer to the tractor and is lighter than the rotary mower.
We ended up buying a new Land Pride RFM 72". It works excellent and I mowed hillside areas that would have been difficult with the rotary (because of the backing and goher holes). We're very happy with our choice. You could probably run (I'm not the expert) all 3 in at least 72" widths, maybe 84" with the finish mower with the pto hp you have. The owners manual has some limits for the implements to guide you, at least give you some ideas...
Do you have all grass? mix of grass and saplings? flat or hilly? etc.