A brief anecdote about shear pins:
Some 30 years ago my Dad loaned his 3 point auger to a young college grad to drill some post holes. He warned him that if he hit something hard it would break the shear bolt, and if it did, he should get a standard grade bolt to replace it. When he returned it, the auger had a brand new PTO shaft. He had put a Grade 8 bolt in and instead of shearing it, it shattered the outer sleeve of the PTO. We teased him about going to college and being educated beyond his intelligence. Needless to say, Dad never had to worry about loaning out his auger again.
Having said that, you should see if you can get an inline clutch for your PTO. It is an option on Bush Hog brand mowers and should be available on other brands as well. It goes between the PTO shaft and the Gearbox and the PTO shaft is shortened, or may work okay on your larger tractor.
They sell 1/2" x 3 1/2" grade 2 bolts by the lb at the farm store. I must be getting better I hogged most of the day and didn't shear one pin. Now if my tractor did not have Live PTO, you bet your bippy I could get PTO clutch. My neighbors father in law told me a horrific story about a guy he knew hogging on a Ford 8N going down hill. The momentum from the hog was pushing him down the hill. He couldnt stop so he jumped off and when he did his foot hit the steering wheel and the 8N and bush hog ran over his feet and cut them clean off. Lots of stories about Ford 8N and bush hog accidents.
My tractor is high profile so even on a slight grade, the tip factor is amplified. I don't like that tippy feeling so I go up and down hill forward, not side to side. I feel it's safer that way. Also when going down hill I can feel the momentum of the hog trying to push me. That's usually when the governor kicks in to try and push back. I now know to push the clutch 1/2 way and mash the brakes which, disengages the transmission to the real wheel. Then slowly let the clutch up and the hog and tractor are playing nice together again. My first reaction was to push the clutch all the way down and this is how I was snapping sheer pins. When I let the clutch back up, snap there goes another sheer pin. I've also started just today using the power director clutch instead of pushing the clutch down 1/2 way. I believe that why I didn't bust a sheer pin today.
Here's how it works, no clutch needed.
There's a lot going on when hogging, you got the PTO shaft spinning and big munching machine spinning behind you, you have to watch where you going, you have to be at the right speed, you have to make wide sweeping turns, your RPMs have to be in the sweet spot, you have to keep an eye on the temperature guage, watch out for animals and fallen tree branches, etc... all this requires your undevided attention. It's no time for day dreaming I'll tell you that. This is my first tractor and first time with a bush hog. so I'm extremely cautious and of course my land is not flat either. The old girl and I are developing a very respectfully relationship. She is strong and I know it and have great respect of her power. Unlike a horse though that has a mind of it's own, the old girl does everything, I tell he to do and exactly how I tell her to do it.
the way I figure... bush hogging with the old girl, she gets about 1 gallon per 1/2 acre and it takes us about an hour to hog an acre. I have to put her to full throttle and hog on first gear. 2nd gear leaves to much standing and I'm snapping too many drive shaft shear pins (grade 2 1/2" x 3 1/2" bolts) This field was last mowed for hay 2 years ago and it's tall and thick. 
