I agree, it's gonna be the gearbox that goes next. Stock A12 4 speeds are SO weak, the first power shift or high rpm launch you do and you actually hook up, it's going to asplode. That's my bet

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.

I think the heart of the person you race will break first....cause they will be devastated by the twin Hair-dryer A15 launching off the line to Victory!

gearbox

seeing as those its a stock gearbox, ill say that.
second guess would be something in the diff, ie axels, center and so on

no takers on the tailshaft?

the wallet went years ago

Just to be different I will say the weld will break first.

Welded diffs can let go at the axle too. I've seen axles let go at the wheel end too.

So if you find yourself skidding along on 3 wheels, you might break something more than the driveline.

So please be careful on the street. There is a reason why locked diffs are illegal on the street...and it's not just because of the skidding noise when you turn corners.

Surely, the wallet will break first

I think the A12 gearbox sounds like the weakest link ,

although after a few attempts at reverse parking your patience with the locked diff might start to wear a bit thin.