M mast on X
It has been done successfully by someone, but a few things to consider.The M mast is taller (30')so your X mainsail (28') will not reach the top. (new sail perhaps)The M rig requires no backstay for rotation therefore the side chain plates are further aft, you will need to install a set further aft.The M mast is beefier & heavier, more weight aloft.The headsails are the same on both boats.It is doable, and the X is fully capable of carrying the M mast & sail, but it does entail some modifications.