Hi bud!
Sorry to hear of your troubles, I know how frustrating it can be, i.e. clutch not working properly etc.
Gearbox wise, James Martin is the expert. With mine, I changed the g/box oil and put mtf in, this at first seemed better, but during the winter it seemed gear changes were very stiff, and at the last service I put Morris oil in, atf dexron 2 in. My god, what a difference!! Lovely gear changes and seems quieter. My 90 has done over 120000 miles, so synchros are slightly worn, hence why I feel such a difference. I change g/box and t/box every 8 thousand miles. It should rumble on forever if I keep this up! I've got ep 90 for the tranny box this time, same as the diffs, as its way cheaper than 75w 90 that's recommended! With this many miles on I doubt it much matters what you bung in it, more that you change it often!
In regards to master and slave cylinders....i had the clutch decompress all the way to the floor, and was just able to engage/disengage gear the last 1/2" of travel. I bled the system, everything seemed fine, the master looked new! Until I realised it had been replaced by a britpart item that was just letting a tiny amount of air in. I renewed both slave and master for genuine and I haven't had an issue for two years or 20 thousand miles.....
I'd also recommend buying an "eazy-bleed" kit, takes all the hassle away from it. Hope this helps/reassures somewhat. Good luck mate!