Don't rule out the Mazda MX5 gearbox. Much better ratios than the Ford boxes and still in keeping with Morgan as the current 4/4's use this box.
Not sure how easy it would be to fit to a Ford Crossflow? The type 9 5spd conversion is quite commonplace (lots of lotus/westfields/locosts have done this) and as far as I've read doesn't require 'too much' fettling.
Have you got any references to a Mazda/Crossflow union? Would be useful to see.
