GA Precision will send you a 3/4 MOA gauranteed accurate rifle. I have had them build me 2 AR10's so far, and both have been 1/2 MOA guns consistently. I don't think you need a 26" barrel, but if you do get one, make sure you get an adjustable gas block to mitigate your dwell time.
Start with a Krieger or Bartlein pipe, and you'll have a tac-driver, especially if built by GAP. There are other great rifle builders who do AR10 work as well, but this is my experience with GA Precision.
My .260 Rem AR10 built by them shoots .4" to .6" 5-round groups all day long at 100yds, and I recently shot a 9.25" group at 1000yds off my truck bed, off the bipod, in the wind, on 10x with my optic, with lots of mirage here in the high desert.