Double or single down tube... too narrow or too wide.. If the opening is large enough to slide the 750 into from side to side comfortably... you can make it work..
Just make wider mounts to support the wider motor and make sure they are braced right..
You could us a frame that has a single tube cradle if you build the mounts  right..
Another thing to watch would be for the exhaust to down tube clearance.. There has to be room to get the 4 pipes by the down tubes..
Once you have the motor centered in the frame.. then center the rear wheel in the frame and then space your sprockets, if you need to..

Build it the way you want it ..... then learn how to ride it like you own it..
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