To be ethical, and all that other garbage, I would stay away from any caliber for ELK that is under .400 in an AR. Now, Mike at dtech builds some WSSM uppers. IMHO, I would check those out:
http://www.dtechuppers.com/ar-15-wss...receivers.html
Mike has a hardcore following, and guys that are buying his stuff are loving it.

Remember that you'll be tracking a wounded elk with a 300wtf cartridge, even with a vital shot. I wouldn't use it, along with the grendel or spc. I'm a magnum guy when it comes to those fuckers, and I know what it takes to make em DRT...Big bullets moving fast, make big holes...I'm reserving my bigger ar calibers to deer, yotes and VC.