hello everybody

my name is pierre, i'm new on this site, i live in france
i've got a yamaha 350 rd aircooled year 1973 and a yamaha 500 rdlc year 1984.
i would like to give to my 500 rdlc a new life and install the engine in a good aluminium frame, so i think the 250 nsr frame is the frame i'm looking for, especially because of the shape of this frame near the kick starter.
i found a frame on this site, but the owner tell me there's difference between mc18 and mc21 frame.
can anybody tell me what are the main differences between this two frame?
it will help me for buying

thanks