I have this trigger in mine as well, although I was able to get it adjusted to a nice 4lb pull with no creep. I posted about it a couple of pages ago with no response, prototype Huh... Who knew.
They are modified AI 338 mags. The feed lips on a 300WM would be too long for the Winchester Action. The mod is at the front of the mag, a notch is cut out.
As has been stated above, Shilen has their DGR made by Stiller. Only down side is that the bolt heads are not changeable. Also much more expensive than a "Savage Donor rifle"
Ideally, a Remington foot-printed Action (ie Stocks, triggers, DBM's etc) with a floating Bolt Head and threaded to accept Savage barrels would be the best of both worlds.