Much as I hate to say it, you'd probably be better off just buying a new Winchester XPR chambered for the .350 Legend or rework the AR with some lighter weight components to get the weight down. At $549 MSRP the street price the base model Winchester XPR should probably be around $450-475. By the time you buy a Savage donor rifle and an aftermarket barrel you'll have more than that wrapped up in it, and then you'll still have to get it to feed reliably which will be a chore. Savage's centerfeed magazine boxes have a big dimple in the sides to support the neck of the stacked rounds which is going to cause problems with a straight walled case.