Righty then!
I need more info!
I've watched a few of the Moss Motors Ltd. YouTube vids about overdrives - simple and informative.
I'm a little fuzzy on the throttle linkage / overdrive bits - is it a physical 'on the dash switch', or an internal one?
Complete new gearbox with OD attached to it. Throttle linkage not touched. In 3rd or 4th you flick a switch on the dash to enable OD which gives a 25% gear reduction. Gearbox swap is same as swapping a normal gearbox. However you need to get your drive shaft shortened if the OD didn’t come with shortened one and you need the dashboard switch and make up the wiring loom for the switch to operate the OD on the new gearbox