brush hog clutch

Is a clutch essential on a 38 hp large compact tractor? I have always used a overrun clutch on my Ford tractors. I now have 2 newer JD'sn - 38 and 45 HP. I have a new clutch on a 60" rototiller. getting ready to brush hog with a 72" hog on some what rough area? thinking of using the roto tiller clutch. locally pricefor a clutch seem unreasoable. farm stores on line have better prices. not sure if there is a detail i am not aware of??? Thanks Bill in SW MI

We need a bit more info.
There are over running clutches like you used on your Fords. There are slip clutches like you see on better models of rotary mowers and there are different types of clutches in tractors.
Do you need an ORC to run your mower?
That depends on what type of clutch/pto you have in your tractor. For transmission pto you need an orc. For independent and live pto you do not.
Do you need a slip clutch on a rotary cutter?
They do prevent damage to your pto if you hit something like a big rock or stump.
Some cheaper mowers just use a soft bolt as a shear pin.
I have never owned a rototiller but suspect they have either a slip clutch or shear bolt in the driveline. A rototiller does not generate the huge inertia that a rotary mower does so you probably do not need an orc on one.
Define your terms a little better and we can probably help.

a 2016 john deere 5045-45HP and a 2017 jon deere 3038e-38 HP. the 2016 is a gear transmission and the 2017 is dual range hdrostatic. IBrush hog is 72"

still have a Ford 8N and a 640.

We aren't getting anywhere.
I'm still not certain what you are wanting to do. Are you wondering if you could take the slip clutch off a rototiller and use it on a rotary mower? I can't tell you without a lot more info and probably pictures too.
Are you still wondering if you need an orc?
Can't tell you that either as I know Nothing about the tractors you mention (except for the Fords). What type of pto do the JD tractors have?
I flunked mind reading class.
Sorry

From this list which PTOs are you John Deeres equipped with?

PTO TYPES
Transmission
The earliest form of PTO is the transmission PTO. The PTO shaft is directly connected to the tractor's transmission. The PTO is only working when the tractor's engine clutch is engaged, so if you take the tractor out of gear while slowing down the PTO will stop working. This is a disadvantage in applications PTO equipment, such as a rotary cutter, with heavy rotating parts will transmit that energy back to the tractor via the PTO shaft when the clutch is disengaged, thus continuing to drive the tractor.

Live (two-stage clutch)
A live PTO works with the use of a two-stage clutch. Pressing the clutch pedal to midway of travel will disengage the transmission, while pressing it fully will disengage the transmission and the PTO. This allows the operator to slow down or change gears while the PTO is still operating.

Independent
An independent PTO means that the PTO shaft is controlled by a separate clutch. The foot clutch has no effect on the function of the PTO. As with a live PTO, this allows for full control over the tractor while separately controlling the PTO. There are two major types of independent PTO: mechanical and hydraulic. A mechanical-independent PTO uses a separate on-off selector, in addition to the PTO control lever. Often the tractor must be stopped or off to change this selector position. A hydraulic-independent PTO often uses a single selector.

PTO DRIVELINE PROTECTION

Over Running Clutch
An overrunning clutch is often recommended and needed with a transmission PTO. Without it, the driven equipment, like a rotary cutter will put a force on the PTO shaft, and then the transmission, due to inertia. The equipment will "drive" the tractor, and you will still move after using the tractor's transmission clutch. An overrunning clutch prevents this from happening by allowing the PTO shaft to freely spin if the shaft is being turned by the equipment.

Slip Clutch
A clutch unit is usually mounted directly onto the input shaft of PTO driven equipment. The PTO shaft connects to the slip clutch. These are often spring loaded and can be adjusted for drive torque needed by the equipment but will slip if the equipment encounters a sudden overload torque beyond that, such as a rotary cutter hitting a fixed obstacle or tiller hanging on a big rock. The clutch slipping can prevent serious damage to the equipment, tractor, and driveline.

Shear Bolt
Simple overload protection. This is a bolt or pin, usually not hardened, that secures the PTO drive line yoke to a gear box input shaft. The shaft will be round, no splines or keyway, so that the yoke will spin free on the shaft if the bolt/pin is sheared by the equipment encountering an obstacle or overload.

If you are asking if the slip clutch from your tiller can be used on a rotary cutter, I would say it is likely it can. However that will depend on the clutch type, gearbox input shafts, torque rating, drive shaft attaching and or length. As Ultradog MN posted more info is needed to give a proper answer.