Without having chamber specs right in front of me, I've gotta guess...so, here goes a WAG. I'm not entirely certain that you'd get a .308 chambered barrel to clean up and give a 'good' 7.62x54 chamber. I know you could use a .30-30 barrel and achieve the results you want.
The SB2 frame will handle the round, and yes, you'd have to modify the extractor. Can't answer your question about stainless/plated receivers and the accessory barrel program.
Biggest question I personally have (and I KNOW there's gonna be a ton of responses that pretty much state I just don't have a clue), is WHY?
The surplus ammunition is not necessarily really good stuff, other than it can be inexpensive. It is loaded with a steel cased bullet that's .311 diameter, is loaded with corrosive ammunition, and the steel cartridge case is Berdan primed.
Wouldn't it still be WAY more cost effective just to pick up a Mosin with a decent bore to fiddle with?
Forgive me, but I still have no use for Commies, nor their combloc crap.