If you are going to build a .318 WR it should be done properly.
If authenticity is not important then consider the .338-06 with 250gr bullets. Much easier to get the components and approximate the look and performance of the .318(.330 bore).

See if you can pick up a clean Husqvarna 146 action from Simpson Ltd.(Pick one that hasn't been drilled).

Classic pre-war commercial FN large ring Mauser 98 action. Highest quality and period correct. Best bang for the buck.

http://www.simpsonltd.com/index.php?cPath=350_370_371&sort=2a&page=1

Husqvarna 146

Original .318 Westley Richards