Thanks for the good wishes.
Ok, here are some pics:
Here's the Sportster seat sitting on the '81 650 custom frame. Not too bad looking.
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Here's the rear fender bolt mount. Might work.
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Issue #1: The Sportster frame is much more narrow than the Honda.
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Issue #2: Because the seat does not rest correctly on the frame, there is no support for the sides of the seat.
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Here's the bottom of the 2 seats. Notice the rubber pads on the 650 seat that contact the frame and the rear steal cross brace that bolts to the frame. The Sportster seat does not have any rubbers because the seat rests on the frame for the length of the seat. Also notice the keyhole notch near the center.
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These next 2 pics show how the Sporster seat hugs and rests on the Sportster frame. Even the oil tank and left side cover help support the seat. They are both made of steal.
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It would take many mods to the seat and the frame to make it work. Maybe there are some guys who have done it. I'd love to see how.