Thinking about the experience I had with my 850, turning a 60' x 200' area, that was once a hedgerow, with some 8" to 10" trees, and lots of brush. I let it sit for a few years, but hooking onto roots from cherry trees and small stumps, had to be careful. Similar soil conditions, rocks concentration of roots, 2 bottoms was all it could handle. I'd be making sure the clutch was good and adjusted in that 841, it could be some hard plowing. It would pull mine right out of 1st gear, but it's a bit worn as is too. I'm also of the opinion that in loose soil or soil that had been turned, the 3 bottom might work ok with one of these,( like in soil pictured below) shallow'er the better on the depth though. I cannot imagine doing 6 acres of those kind of conditions as I recall turning that fresh ground for a deer plot was hard on the 850, and for scale our 7 acres of corn was next to this patch I did, so I can visualize 1 less acre of tough ground, that is quite a task for one of these tractors, also have to watch spinning the tires in rock, can tear em up after hookin on to a root or something. When we used to clear big sites, usually a clearing contractor would come in with a buncher-feller, shear off trees, and process, or loggers would clear cut. We had a pair of 235 cats on stump removal, and then hit the area with the D8K's with root rakes, to clean up before we hit it with scrapers to pick up the top soil, before mass excavation. Here is a photo of what one looks like, it's on e-bay right now, and I believe would fit my D7's c-frame angle blade, same guage as the d6 at 74" and it's what I would use after clear cutting and I would love to grab it, it's one state over, could put it to good use here. Lots of heavy rock would be hard on one of these, you can see a few tines bent a little, but they sure do work nice. If the ground is really tight with roots, a multi shank ripper on the back of a crawler, similar to a subsoiler, but more shanks, would break up the soil nicely, then hit with a root rake, then try plowing. 
I did this area for my neighbor, it was the previous neighbors garden for 40 years, replanted it in grass, inthe fall, with rye straw mulch which actually came up the following spring, like I planted it on purpose! Real nice soil, but tough sod on the first round, this Harry Ferguson 2 bottom, sunk in deep and turned over some furroughs, this was the 2nd time I turned it after discing once:


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