Has anyone ever changed a 1977 4 speed model X1/9 gearbox for a 5 speed gearbox later model. Can I do a straight swop? Does it fit? What mods are needed?

I have done it but used all 1500 parts, ie starter, flywheel, clutch etc not sure if you can do it without changing those parts. Someone else here might know.

The big question is do you already have 1500 rear arms - if not then you need to get a pair in order to accommodate the 5-speed gearbox.

Some later 1300's came with the 1500 arms but on a '77 it could easily be proper 1300 arms.

In terms of the gearbox parts (I've never done this so please double check) the input shaft spline is the same between the 4-speed and 5-speed so you should be able to retain the flywheel, clutch and starter.