Well, as predicted, I am having some fun and games with my build.
I made the possibly foolish and completely reversible choice of using an external BB, for no other reason that I have some here. I then fitted a SRAM Rival cx crank and found I needed two spacers to prevent the inner chain ring from clashing with the chainstay. All good, thought I, until I finally got around to tightening up the cranks, at which point the penny finally dropped that with the spacers fitted the BB is effectively 73mm wide, and road cranks are 68mm :facepalm:
Now onto plan B, which may or may not work, depending on the spacing between chain rings of road vs mtb cranks. More later if it works (or not).
This is quite an entertaining build, in a beating your head against a brick wall kind of way.