I use grade 8 bolts from Tractor Supply (the gold ones) and go thru lots of these with my Ford 3 bottom. Buy them by the pound. I know your plow is different, but generally, same concept. Its a fine line between having a shear strong enough to allow the plow to work, but weak enough to prevent damage when an obstacle is hit. I break lot of shears, but it is dependent on the soil and where I plow. It can be frustrating. But the more often the area is plowed, the less likely you will hit something. My advice is to use grade 8, you could drill it out and go up a size slightly, but that is a risk. But go slow, keep you hand on the lift lever, and you foot near the clutch and be quick if you feel resistance. Start out shallow, especially if you are in an area with rocks. And wait for some rain to soften the soil.
I had a plow without any shear protection, and the first time I used it, I broke 5 grade 8, 5/8" bolts holding the frame together in one loud pop. I replaced all of these and the second time I used it, I broke them again and also bent the crap out of the main frame. Needless to say, that plow was toast. My point is, some soil is just too rocky to plow easily, no matter what. It can be done but will take time to work the soil, to find the rocks, and to improve it yearly. And keep a pocket full of grade 8 bolts in the meantime.
You could also use some other implement first to rip up things, like a box blade with rippers. Anything to loosen up the soil will help you.
