replacement gear box options

I have a series 2 that's running 1980 range rover running gear. Not one for the purest but I like it! It has a four speed manual with fairey overdrive. Basically the gearbox is shot and I need to replace it. I would like to fit a five speed but need advise on my options please.
I am very new to Land Rovers and forums so any help and advise is appreciated.

Try the range rover forum. Basically tho your running a 3.5v8 I take it? Loads of options there but I would have a guess that a later rrc box and trans case based on an lt77 would fit but not sure if your props etc would be right. Could fit a zf auto or an r380 plus case from a disco 3.9? Try the v8 forum too.