The 300Win mag is a great round capible of making some realy long shots , the 7mm Rem mag is another good choice.
Not sure if you reload or not but their are a bunch of wildcats out their that will fit your gun with no to minimal modifications. I'd get a good barrel with a heavier conture and have it chambered and fitted buy a good smith and pretty much any of the mags will get you into the LR game with good bullets.
As for smiths , I just got a rig in today from "308 Nate" and I 'll say that his work looks and feels as good as any I have ever handled and that includes guns built by Darrel Holland and Charlie Sisk. If it shoots as well as its put together it'll most likly be my most accurate gun. Thanks to the US Postal service's "great" handeling it'll be a while before I get to shoot it as it has to go back tomorrow for some finish repair.
Nate's web site is www.tiptopwebsite.com/ssgcustomrifles