howdy macsen .....
Im not exactly sure what you mean & I want to help .
I figure your trying to match Duke Johns mounted Greeks onto vanilla MTW horses ? .... but you lose me when you mention rescaling & rectangles .
Let me know ....
I have some ideas & they seem to work (640x640) no rescaling or rectangle dramas required & all the better if a 768x768 is reality .
SO ... We have options
cheers