My initial view would be Don't do it, if I read your Post correctly you mention the opportunity to clean swap a 5DMK III for a 1DMK IV, so my view would be, don't do it.
I own all 3 Camera Bodies and use them mostly for Wildlife (+ the 5DMK II) I never sell a Good Camera, it's like selling your children.
When I was using the 5DMK II + 1DMK IV, the 1 DMK IV was the King for me, 10fps, weather sealed, much better AF System than the 5DMK II, and the big big decider, 1.3 Crop.
The only thing the 5DMK II had that was better, was a larger MP file.
But then along came the 5DMK III, suddenly I was finding I was using the 5DMK III more than the 1DMK IV, the 5DMK III had a 61 point AF system, larger file size, Ok weather sealing, the 1D MK IV became the back up to the 5DMK III.
Then along came the 1Dx, I bought 2 Bodies, never take the 1DMK IV anywhere anymore, and use the 5DMK III as a back up.
But, this is wildlife Photography, with the occasional Landscape thrown in, street scenes if I'm stuck in a City between flights to another place that has Wildlife, and if a slow Bird flies past, some BIF Images.