Can wither skeletons spawn on any nether bricks?
Solution 1:
Wither Skeletons spawn anywhere within the bounding box of a fortress regardless of the specific solid block.
Solution 2:
Wither skeletons can only spawn within the immediate area around Nether fortresses, so you can't take the bricks and place them somewhere away from the fortress, and still expect them to spawn on those bricks since they're too far away from the Nether fortress chunks.