They make a kit for the 8120 that was standard on the 8010 . It will install the little back door over the power spreader. It works nice and make sure you drop your knifes and slow it down. You can also just raise up your spreader and drop your pan and do it that way also.
To keep from filling house yards w/ cobs I raised the rear door on my 7010. The new xx20 bines only have the door if ordered as an option. As an attachment, there are 2 kits for the 20 series. The first one [I put this one on my 7120] is a couple hinges, a open latch and a closed latch. About $90.00.
The second one [called a windrow kit] includes the above stuff but adds tin work to the inside to funnel stuff better. About $800.
I put the cheap one on and it works great. Couple seconds to open or close. A lot faster and easier than lifting and lowering spreader.
For large acres of lets say wheat, u may want the expensive full windrow kit.
Unless they have changed something from the 8010 you don't need anything to drop straw with the 8120. Slow your chopper down with the clutch thing on the side of the machine (which could be a pain in the a** if you haven't moved it before) and move the spreaders either up all the way to drop straw and chaff of tilted down to drop straw and spread the chaff.