On the wall that runs parallel with the floor joists, I have about a 15" gap between the concrete wall and the first joist. The joist is also about 8" lower than the sill that sits on top of the concrete wall. I am framing right up against the foam but I'm not sure what I can attach the top plate to. My first thought was to attach 2 2x6s to the concrete wall which might give me enough room to nail in a top plate.
Now on the wall that runs perpendicular to the joist, the gap between the end of the joist and the wall is only 5", but that is still too far to attach the top plate to since the foam is only 1/2". My thought here would be to run a 2x6 across the top with half nail to the joist and half hanging off to attach the top plate to.
You dont need to attach your top plate to anything. Build your wall to the top plate comes even with the bottom of the joist. Then drywall back the top of the wall with a 2X hanging over at least 3/4 in. Block from the top of the drywall backing to the floor joist every 6 ft. These are just firring walls so they just have to be attached so there straight and solid.