Your kubota m8540 is rated to lift 4600 or 7000 lbs 24" behind the 3pt lift arms. You should have the lift for the 14K rated GN unless you are very front heavy on the trailer. I have a 3pt GN adapter I made for my Cat II tractors and one for my Cat I tractors. Regardless of how HD the 3pt GN adapter is made, you need to consider 2 other aspects. I use mine mainly to move trailers that are empty or lightly loaded. If you are pulling hard on the GN, remember the majority of the pull is on the 3rd link and not the lift arms. Don't get too crazy and pull the attachment site of the 3rd link to the tractor apart. You are considering a loaded 14K lb trailer with bales. Drop the trailer tires into a ditch at some speed and the 3rd link may destroy the attachment to the tractor. A second point, is going up hills. With heavy weight on 3pt, the pull on the GN adapter can lift the front end of the tractor up fairly easily if you don't have a lot of front end weight. Is your Kubota big and heavy built enough for these tasks? Your M8540 weighs about 5500 lbs (IMHO a little light in the weight for 14K lb trailer) and I don't consider the Kubota's to have a HD 3rd link attachment point. If you have a FEL, the tractor will handle the weight of the GN much better. I did have my prior JD 5300 MFWD (loaded tires and FEL) which probably weighed 7500 lbs on a GN 12K dump trailer and had to be careful of what and where I was going. Lifting the trailer with 3 pt was never an issue and I had less lift capacity than your Kub. I currently use a 125 hp tractor weighing nearly 11K lbs and the trailer is not much of an issue except, I could bend or destroy the GN hitch if I pulled too hard.
I do think you will be ok with the m8540 using a FEL, 14K lb GN trailer and not getting too aggressive-provided your ground is not hilly.
There are dollys (attached to drawbar) available for GN trailers. However, pulling a trailer with dolly could be an issue if not enough tongue weight and going down hill or stopping too fast.