Hi Guys,
Thanks for the welcome 'mintcherrie' and thanks for the replies. Yesterday I called into a local indepent who specializes in LR parts he said 'if the sufffix letter is the same then yes,' otherwise you have to take the input saft out and count teeth (which in itself is a big job), because the ratios were changed several times throughout the LT77s developmental life time.
He gave me the number of a man that would know, he said the same thing, having done this trick several times (important to me, because he talked the talk and has walked the walk, everyone so far has only talked about the theory). In his experience it is a make do and mend for extreme situations like outback expeditions as a means to get back civilization. He also said that, you nearly always get a whine, because of the differential in wear between the original and new parts.
So all things considered, I think I will nick the gearbox out of the 90 till after the summer holiday and probably do what 'boydy1989' suggests, when I can do it at my leisure and have the time to get it right.
Thanks for your help guys, I hope this info is of help to someone else, have fun with your Landys.
Conrad