7mm bl. to a .458 bl. on a Savage might be as simple as replacing the barrel. There was a guy on a site I frequent, who had 3 Savage rifles, with several bolt faces, and over .30 different rifle calibre barrels for them. 3 actions, over 30 barrels - just changeable on the kitchen table once you get the nut off and first change-over done.

Headspacing is done with actual ctgs. for that rifle, winding the barrel in until it touches with a chambered case, then lock the barrel nut - done.

The company selling pre-chambered and threaded barrels for Savage rifles used to be www.sharpshooters.com or something close to that. Texas, IIRC.