Conversion is easy, you need a hole in the transmission tunnel for the gearstick and sort out pedals and clutch and that's it.
 
Column shift auto isn't difficult to operate, just shift it into D for 'drive' and you're away.
 
Tailshaft is plenty strong, it's the diff centres that tend to go. Even a heavily worked a12 will pull one apart:
 
http://datsun1200.com/modules/newbb/viewtopic.php?topic_id=42250&viewmode=flat&order=ASC&type=&mode=0&start=180

You can use a H150 from the 120y (not Borg Warner type but the Japanese one) or Sunny B310 H150 centre in the H145 housing which will take a bit more punishment.
 
Other option is to run 12 inch tyres so when the power goes through, the only thing that breaks is traction.
 
Other option is a complete diff upgrade. Shortened diff or custom offset wheels, re-mount/re-arrange mounts.

I just did one on my 1000, from colum shift auto. The only problem I had was the clutch pedal. I used a 1200 brake and clutch pedal, and the 1200 clutch cable. I had to shorten the housing for the clutch cable on the firewall by about 20mm, so I could get the pedal position and necessary play on the thrust bearing. I still have a slight problem with the clutch pedal sitting much highter than the brake pedal, also you will need to put an other hole in the pedal box to alow for the extra return spring. Otherwise pretty simple jumper wiring to get past the inhibitor switch and wire in the reverse lights. Also the gearbox mounts don't even come close to lining up. You will need to make a couple of brackets and bolt through the floor pan with huge body washers inside the car. Handbrake cable got in the way for me, so I just backed it off a bit. In all was done in a day so pretty simple. HAVE FUN WITH IT!!!!!!!

Prefer auto on the colum anyday! Manual transmission are overrated.