I have just done a Auto to manual conversion, it was easy - manual to auto conversion would be a LOT harder - you would need an auto electrician to add the wiring that is reqired for the neutral safety switch.

I still have the auto, however I suspect the rear band was slipping - slow as molasses in june, in 1st and 2nd until at operating tempature then it ran like clockwork however I like the F4W60L better as more power to the back wheels!

you could use a 180B station wagon as the donor - L18, 3N71B gearbox and (I think) a H190 diff!- the brackets are already on the chassis rails!