Bad choice for a replacement if you ask me. Leaving aside the flatness, 24" tubes have 60cm screens while 25" tubes have 63cm screens. You will not be able to mount the former on the frame of the latter. A 25" big red should have a Wells Gardner K7000 chassis. If it's broken get a replacement chassis (they're very common) or get it fixed. If the tube has burn-in, get a 25" RCA or Zenith TV and swap the tube.