There is a guy who sells new Remington straight stock sets for 1100s on ebay - I got one for my LT20. You can use an 1100 buttstock, or you might contact him to see if he can get one. Also, sometimes the straight stock sets show up on ebay or GunBroker. They usually do not go cheap. You can use an 1100 LT20 straight buttstock. The fore end wood is different if memory serves on a Special Field 870, but I am not positive.
I have modified a couple of pistol grip stocks to a straight configuration, but not on an 870 so I don't know if you could or not. I like the pistol grip on a pump or waterfowl gun where I might get in an awkward position and need the grip more,, but like the straight stock on a light field repeater or SxS.
Good luck.