I think the 1200 gearbox will go first thats if you are using the A12 gear box not the A14 smooth case box as that is stronger.
I Have a locker (mig welded centre) in 1200 sedan. i think they are great. Only prob is when i first got it with a shell i got for free the guy had blown the pinion out of the casing (alloy casing). I then took the crown wheel off it and matched a crown and pinion from a steel casing and put it in that. Next problem since then is it has been through 3 axles 1 tailshaft and recently destroyed a pinion which destroyed the crown. Now all changed over into a alloy casing now as that is all i had left at home so the centre is very strong! Thats a standard 1200 centre! i have pics and will put them on this site when i can.
The reason my gearbox hasn't blown is that it is a A series 5 speed mean't to cope with the A15.

The vannett diff would be a lot stronger and i think you will have a fair bit of trouble breaking a welded center (as long as it is done properly) and the axels are a bigger so that should be no worries. Bigger uni joints in the tail shaft would be recommended maybe out of a bluebird, but from my experience i say the gearbox will go first and i will pick 2nd gear to stuff up first.