W123 coupe 280 CE manual 4 speed to 5 speed conversion?

[madras]

I know there are merc forums but dont have any logins and just happened to be here (and I had a look and couldn't find a definitve answer)

should be a quick one if you know, going from a 4 speed manual to a 5 speed manual on W123 280CE, do you need anthing else other than the gearbox, was the final drive / rear diff the same?

Thanks! Slave cylinder appears to have gone so need to take the box off anyway and fancy getting the longer 5th gear for motorway.