Was this something shot before you knew their shooting and delivery specs? For example, a show that was acquired by FOX? Because you can use that to explain that your footage isn't 60fps. You can make it 720p 60...but there will be repeated frames. Can't avoid that.
What I would do is make a ProRes 720p60 sequence, copy and paste the sequence into it...apply a smart Deinterlacer (nattress filters do this)...render, and then export. Now, do they want Op1A MXF in the XDCAM codec? Or do they want a QT file?