Custom rifle. Your best bet is to buy a 700 in 8mm Rem Mag(if you can find one. 700's in 8mm Rem Mag are only available through their Custom Shop) or a 700 in .375 H&H and have it re-barreled. Won't be cheap. Mind you, you might find one already in 7mm STW, but there won't be many.
Make sure you can easily get brass and/or ammo, locally, first.