How to insulate sloped top of concrete block foundation?
I've had an almost exact scenario.

you basically have 3 options
1) spray foam the entire system. Expensive for small areas. 
2) 1-2 inch foam board that's cut out exactly. tuck tape the seams to the joists/ceiling. This would be a pain to mitre. (I don't think they're meant for this)
3) batt insulation with 6ml vapor barrier over, then tuck tape the seams to joists/ceiling.

Each have pro's cons. spray will be best seal. Batt insulation would be easiest to cut/work with and seal.