Sukee, I did that exactly! I owned a 1995 JD870 and loved it. I bought it new and had just about every attachment you could plus the backhoe. I did tons of work with it and it always was able to do the task at hand. There were a few years that I never missed a day on it. That machine never let me down and the few times that anything broke on it was because I broke it.
I always wanted a cab machine, so when the Deere's came out with a cab I was really excited. Finally I saw a 3520 w/cab on ebay with only 7 hours on it. It also had a 300cx loader,fertilizer spayer,cone spreader and loader mounted blade attachment. I figured I could sell all the attachments except the loader and recoupe some of my money. I ended up winning the auction and driving to Illinois to pick it up. Everything was like new................well it was new!
Got the 3520 home and loved the cab. Hated the ehydro and stability. Almost upset it on a cross slope that my old 870 went accross without even thinking about it. Love the cab.
Anyway I finally went with some Unverferth wheel extensions to get a wider rear wheel stance and improve stability. That did help, but its still no where near as stable as the old 870 was. The 3520 also does not ride as nice as the 870. I'm not sure why, but the 870 did seem to follow the ground better. Although I love the cab, I really don't like the tractor, but I'll have to live with that. Before you jump into buying a 3x20 series I would strongly recommend that you try one out at the dealers where you can put it on a slight side slope. You will quickly see the difference between the 870 and 3x20 tractors. Go slow and be careful and don't let the dealer tell you it's just the way it feels cause your not used to it. It's not. If you are close enough to me come try mine out.
I ended up selling the 870 on ebay and have missed it ever since. The 870's are one h3ll of a tractor!
New tractors are nice, but sometimes not better. I do love the cab on the 3520!
One other thing...........I think if you hooked the two together drawbar to drawbar the 870 would have drug the 3520 around all day.
Here's the 870 with 2018 hours on it up for sale on eBay.
Here's the 3520 with the added 8 inches to the rear with the wheel extensions.