Hey, Wannashootit was warning you off the .585 338 Lapua, not the .532 (ish: I think it's actually .534 or something) 338 UltraMag. Or you could look at the .338 Edge, which is the .300 UltraMag necked up to .338. The .338 UM is a little shorter case to allow using longer bullets. It'd be my choice with a Remington long action.
Gunsmith can open your bolt face and put in a Sako or M16 extractor, as stated above. Or you can buy a new bolt from Pacific Tool and Gauge. Might have to have some feeding work done.
HTH