Is this the same action as a savage? Will the barrels interchange? Will aftermarket savage stocks work?

I was told that all parts will interchange. It is just a Savage 110 without the accu-trigger. Good luck.

Does anyone know if the leupold 2 piece scope mounting system will work on the stevens rifles?

yep, it's the same ole savage 110 action. The savage 110/10 bases will fit, just make sure you get the new type bases for the rounded rear receiver.

i suggest you join this forum. it has TONS of people who use all kinds of savages. I'm in the same boat as you looking for two piece mounts for the stevens 200. there are tons of savage and stevens users on there who have it all figured out.
www.savageshooters.net/forum
good luck

If anyone is interested in trying to mount a Nikon scope to their long action Stevens model 200, here is what worked for me.
Leupold STD Mounts #55742
Leupold STD Rings #49901