Just going to post a quicky tip that most of you probably already know. Ignore it if you do. Mossy's are aluminum in the receiver. Aluminum and steel screws/bolts are a bit diff than other things. Steel screws up aluminum if you're not careful. The easiest way to be sure is to stick the screw into the hole and slowly turn it BACKWARDS til you hear/feel the tiny *tick* as the threads line up. Only when you feel the threads line up do you then go forward. No cross-threading that way.
On the stock, when removing or replacing one, take the trigger group out and all the loose parts, too. The trigger group locks the stock in place and prevents it turning with the lug on the back of the group. With the group out, you can then turn the stock or grip a quarter to a half turn to let off all pressure on the bolt and then it'll remove easily. If you need to, stick a screwdriver or an allen wrench or whatever yours is into the bolt to hold it in place while you turn the stock. When reassembling, have the stock 1/8 turn out of line with its place and then TURN THE STOCK to seat the bolt solid. Prevents stripping. That stock bolt does not have to be gorilla grip, incredible hulk tight. The trigger group lug locks it into place.
On the receiver....head to gunbroker if you need one or if you just want to do your own build. A brand new 500 receiver is $35 plus FFL and all. I've bought them for folks who simply wanted their guns drilled and tapped. Buying an entirely new, already tapped receiver is cheaper than drilling and tapping an old one.
. I just need to modify the Magpul forend I picked up. They have to be heavily modified to work with the short action tubes.