to make a smb3 bowser boss you need to find the block in the smb3 special block section (the block your looking for is next to the note block (block with a note on it))
Yeah, you need to find in the SMB3 Blocks and Tiles Category under Special. There is a brown brick tile specificaly designed for the Final Castle in SMB3 and it is only destroyable with Bowser's Jump Attack by smashing on the Ground.
Also, just as a little notification, there is a 4x4 Block sized SMB3 Final Castle Brown Bricks under Misc. Section but that is not smashable what so ever by Bowser's Stomp. Only the 1x1 Block Brown Brick will do it correctly.