There shouldn't be a problem with that. A low kV motor on a lower-than-optimum voltage would be fine. About 18S would be more appropriate!
The only thing is, as you rightly say, the amount of current required to get it moving with the correct gearing for 40 mph will be huge. The lack of voltage to get the same power output, means the current required would be massive! I doubt that your 25C 3600mAh cells will cope.
Gear it for about 15mph, and give it a go.
The other problem is that the cogging will be terrible due to the low rpm. The reason the 28k-35k rpm guide was made, was that sensorless ESCs and motors require this kind of rpm to minimise cogging.
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