I've seen a Cannondale 650x650, horizontal top tube, frame size would have been about 56-58cm. Might be worth a search.
Edit: Apparently, they only came in sizes to 52cm, the long head tube is deceptive. Nonetheless, stick a long seat post in it and it could be just what you're looking for http://www.bikepedia.com/Quickbike/BikeSpecs.aspx?Year=2002&Brand=Cannondale&Model=Multisport%202000%20Si%20%28650c%29&Type=bike
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@gbj_tester started
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I've seen a Cannondale 650x650, horizontal top tube, frame size would have been about 56-58cm. Might be worth a search.
Edit: Apparently, they only came in sizes to 52cm, the long head tube is deceptive. Nonetheless, stick a long seat post in it and it could be just what you're looking for

http://www.bikepedia.com/Quickbike/BikeSpecs.aspx?Year=2002&Brand=Cannondale&Model=Multisport%202000%20Si%20%28650c%29&Type=bike
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