The interesting thing is why you'd want to change to manual ?
I have driven all boxes & engines in RRC's and the ZF with a BW is about as nice as they come.
Like Mr. getlost mentioned, oil needs changing in shorter intervals then reccomended - and use good quality oils & filters and they are very nice drive lines.
Converting is not difficult, just a lot of work : pedal box, tunnel, flywheel, clutch etc. to mentiona few things.
Doable, obviously.
But in original set up.....