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I had considered harnessing the expansion that causes the pressure, but at the same time I am hesitant to consider it. I'll get to it in a second.
The most opportune place is the hottest place for a stirling to run and thats right out the block, but I probably at best would only be able to get one small unit at that location right now. Insulating everything would be advantageous if you wanted to take advantage of trunk space and put them there for ease of access and mounting.My muffler usually takes a bit of time to get hot but once it does it's scorching. I burned myself rather badly just leaning into my trunk before I realized my leg was touching the muffler. . . it protrudes slightly.
To the pressure/expansion/turbulence. . . I considered it but I'm not certain it actually saves you energy. It would be like running a turbo or supercharger. You can gain more power but it comes at the cost of the engine having to fight the backpressure. In the case of the turbo you typically lose fuel economy(efficiency typically means power/weight and thats always a gain in the case of more air and fuel in the engine). The reason economy usually goes down is because the engine, standard ecu, will inject more fuel to match the greater airflow. Usually this exacerbates the normal inefficiencies, rubbing two pieces of metal together wears them down and rubbing them faster usually has an inceased effect kind of thing.
All that said I tend to lean away from the turbo and towards heat recovery because the kinetic energy of the air from the exhaust is not immense, but creating back-pressure is a huge problem for the engine. The Stirling would cause the air to become more dense and slow down but not enough to cause a measurable backpressure.
Another point of consideration would be creating ramscoops for the stirling cold-sides. the colder the air, or the more cold air, you can get the faster, or more heat you can convert to motion. Effectively the stirling could become much more efficient at highway speeds since it follows carnot ideals rather closely((700-430)/750 or (700-343)/700 degrees input-output/input in kelvins).
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