Hello Guy, I think you've got it in one!
I think the reason why the BOL does not do transmissions is that most require a fair number of jigs and service tools along with a wide selection of spares to draw from as you may need to cross match components to ensure tolerances are correct.
Gearbox building is a 'Black Art' best left to them that has all the required skills and bits. You would not want a box of false neutrals, would you?
If you do feel so inclined, you would be better getting the service manual, even a copy on hard disk would give you a view to the scope of the subject. Modern gearboxes are a little more complex than the old three and four speeders with just single input and output shafts.
I was not aware that Haynes ever did anything other than these weekend DIY brochures, for simple repairs.
Handyman,
