1.) You'll definitely want the 20MOA rail for 308.
2.) It's an EGW rail that's around 6.25 inches long from end to end. I'd say go with the middle one.
3.) I have a vortex 6.5-20X44 scope with a 20moa rail and my rings are the vortex tactical lows which are .84 inches tall and it still has about a quarter inch of clearance from the barrel with my Butler creek scope covers. So the smallest rings would be fine.
And FYI. You will not get a good cheek weld. You'll get a chin weld. You'll either have to order a new stock or get one of those kydex adjustable cheek rests like I did.