early 70's honda 250 to 500 conversion

 I've heard that putting a 500 motor into a 250 frame is a favorite
swap for some flat-trackers. I'm wondering about this conversion. I
have a 1975 Honda XL250 frame. Does anyone know If an early 79-81
XL500 motor will fit into this frame? or what mods might need to be
made? any helpful comments would be appreciated.