About the WSM.... You would need a new bolt head like you mentioned. Probably a new firing pin assembly. If you didn't remove the barrel the smith would do that and then reheadspace it for you. Now we are getting into the $200+++ range. You would need to buy a magazine or modify yours. Your twist rate is probably not optimal for heavier bullets but would do ok.

Or you could buy a Shaw barrel from Gun Shack right now for 158 + TT&L, buy a short action magnum bolt assembly, read up on barrel removal and headspacing on this site. Go shooting.
http://www.gunshack.com/index.php?ro...product_id=183

Anything with a 308 based case is a much easier swap for you. 22-250, 243, 260, 7mm-08, 284, 308, 338F, 358W. Those last two have some knock down power but not considered to be 300 yard calibers.

Good luck