Are you wanting to go with something oriented toward tactical or more similar to factory? Pacific Tool makes a very nice factory replacement. I just got one, and while I haven't fitted it yet (waiting for stock to arrive), it appears to be top notch. Williams makes a very nice steel replacement for about $155. Most of the others are quite a bit more.
The one thing a long range 300 RUM nees is more magazine length for handling heavy low drag bullets. Check out the replacement metal made by Wyatt's. It allows feeding 300 RUM ammo with up to a 3.80 OAL in a Rem 700 LA. Midway sells them. Some smiting is required.
I agree with LouBoyd, the Rem 300 RUM needs a longer mag box to maximize bullet seating. Does Pacific make a longer mag box? I would go with whoever made a longer mag box, which I probably will do at some point.
Wyatt's makes an extended magazine box that will work with factory, Williams, or PTG bottom metal. H S has a detqachable that doesn't inccrease your mag length. For the tactical style detachable with extended length(3.9)
look at Seekins, very nice quality!