Opening up the FH to accommodate the larger 5/8x24 thread pattern may be the least of your worries.
First and foremost...I have never seen a Noveske 11" .300 BO barrel...they are generally 10.2" or they go up to 12.2". If that's the case...then the typical 5.5" Extended FH won't get you up to legal lengths for non-NFA purposes (you'll be ~0.25" too short). Before you do ANYTHING, I'd be damned sure what length the barrel is specifically and plan accordingly. Also, I won't get into the legalities of even possessing the upper if you don't have an SBR lower to go with it, but if you do get it, I'd have it out the door to whomever you decide will do the gunsmithing within minutes of it arriving.
Best route to go if your barrel length is sufficient enough to work with an extended type FH is to get one made for your barrel's 5/8x24 threads and sell off the .223/5.56 one to pay for it. If you don't want to go that route, check with Steve and the guys at ADCO and see if they'd even be able to open up your extended FH's threads to 5/8x24 and what they'll charge for the service. If they'll do it...I'm guessing it'll be ~$100 including permanently installing (pinning) the FH to your Noveske barrel.
