chrisgrim, I really roughed up a concept in Zbrush, based on the Farmer description, which has a bellows upfront containing lungs and heart, technically the nose and mouth would have to allow for this as well, say Mick Jagger or Stevn Tyler. 
I increased the horse mass and reduced your man mass.
Another approach would be the giraffe physique if you wanted legs a little more spindley. Giraffe have more dog build, massive front, lower but less powerful rear (akin to many dog breeds) and in the front are massive lungs and heart to get blood to the upper extremity. In fact this would be closer to what you want even though the body is shorter than the neck. The upper body does require a lot of blood and air though.
Unless total fantasy is the goal, then all this nonsense is moot. Hehe. 
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