True Powderman for the 308. Although not to many shoot 1000 yards in FTR. I shot FTR last year and we shot 600- 90% of the time and a few 800 yards. The majority of the time it is 300 and 600. As you mentioned a 20 scope base would be required. I shoot a 6MMBR and have all the room i need to get to 1000 yards with my Sightron. I wish i could afford a Nightforce,but budget say's not yet
. Being FTR is limited to 223 and 308 and the fact that you shoot 300 and 600 yards almost always the scope would be fine for what he will be doing.
As for scope choice both Weaver and Sightron in straight 36 power is ok. The one issue you might run into with a straight 36 is Mirage, you have no adjustment to help minimize it with a straight power. I would go variable if possable. What i see ( with my eyes). F-class Open and FTR seem to be controlled by variable while Bench seems to be straight power.