This should be a very interesting build, I will definitely follow it.
I like your thinking with that 18t child's chain ring. As far as the Brooks B72, you'll be surprised how comfortable it is, one can go on for hours with out it being an issue.
As far as handlebars, 7/8" moto crossbars are a great choice for aggressive riding.
https://www.btosports.com/p/renthal-offroad-handlebars
650b's should be very interesting, I have never seen that done on a Klunker build.
Klunker below is still going strong.
http://thecabe.com/forum/threads/newly-finished.19600/