You are right. It can not be done. You mention that the chassis are diffetent. That is right = much different. The MX4500 & MX5000 are a Thomsen "box" chassis where as the MX5500 are Bang & Olufsen's own and own built single pcb plate chassis 39xx. Also inside the television there is not the same room, and the picture tube are not alike.
Note that the LX5000 are not simuler to MX5000 (diffenent age).