I would definitely not do one in a SA, so your heading in the right direction. The 700 LA is longer than needed but should work fine. It's used for the RUM cartridges which are very close in diameter to the WSM's. The WSM has a sharper shoulder than the RUM, 35 vs 30. Best thing to do is get an opinion from your smith and find out if he's done many WSM's.
The Howa action is about 3.57" long and a 700 about 3.71". The WSM is about 2.1" long which gives you about 1.4" of bullet nose clearance and with some of the longer bullets, you might need up to 1.2"
I find this web site very handy for making case comparisons and judging what case will work in what action.