I just wanted to update anyone that comes across this thread:
I got my parts kit in, and started stripping my receiver for Cerakote. I noticed that there were remnants of a piece of plastic at the magazine's entrance to the receiver. Sure enough, Winchester used a plastic magazine throat on these guns and I thought I would pick up a new one. Surely they make this critical part.... Right?
Nah, man. NOS parts are the only way to get it and the 20ga is out, gone, nada. I can get these parts made in aluminum or delrin, and I might do that just to offer them to people trying to resurrect these guns, but the reputation on these guns is so poor I'm not even sure it's worth it. I'll model it before I risk breaking it, but jeez what a poor design.
Hell, my parts kit has one and I'm still regretting starting this project.
If you need a magazine throat for a Winchester 1400 and have the remnants of your old one, PM me, but think about the ramification of this before you buy one of these shotguns.