Iain, a thread on the spindle nose, and the MT could / should be left until the headstock is finished and attached to the lathe bed.
Be looking through some of my old books, I have 3 from the 1920s / 30s "The Amateur Mechanic & Work" Hand Book, "The simple Lathe and it's Accessories", "Small Lathes, Making and Using", and "Metal Turning Made Easy". In the second book there is enough info, drawings etc to build a 2.5" back geared lathe, including how to make the patterns for the bed. About the only thing I'd change would be to enlarge the spindle diameter from 1/2" to 1", and use angular contact bearings, I think the original spindle run in the cast iron of the headstock casting, this would be fabricated as you are planning to do. This was the only way a lot of the early Model Engineers could get a lathe, and much good work was done on them.
Ian S C