I think that we will have a very hard time to prove that he could not have been of a BMI of 17,3, and still seemingly display health. As I said earlier, many a very thin person are perfectly sound, healthy and cheerful.
The Victorians, by the way, would not have cared about BMI - but that does not mean that Evans/Fleming would have appeared thicker since he WAS a Victorian. He would have been, as you say, skeletal. And the broader and heavier his body-frame was, the more obvious it would have been.
In conclusion, the only things we can deduct from trying to fit an eleven stone weight on a 6.7 body, is that we have a weight that seems not to correspond with the length. And that means that we may have to add a healthy measure of hesitation to the figures in the records, as far as I understand. And, as has been stated before, the figure 6 in "160 years of madness" only adds to that hesitation.
More information is needed before we can put this one to rest, although until any further development casts light over all of this, I can easily understand those who swallow the measurements in the records with no hesitation at all.