A stock A340 cant take much more than about 200kw at the rear wheels before slipping clutches, so I guess it depends on how much nitrous you are running. They make around 150rwkw stock. A 50 shot should see you at the limit.
2nd Gear sprag will generally suffer on heavy throttle in the 1-2 shift.
Heat is going to be the killer of all autos and the A340 is not known for its strength, so I would suggest you consider upgrading the oil cooler, shimming the accumulator springs & winding up the pressure regulator.
I will doing these mods to mine, prior to converting it to a T400. How long it lasts, though, is the question.