The basic idea behind tightening any threaded fastener is to stretch the bolt so the applied load in the assembly exceeds the expected force the system must retain. The axle is a 1-3/8" OD fine-thread fastener. 400 ft-lbs is the minimum torque to stretch the system sufficiently to withstand 350-400 horsepower the engine is sending thru to the tires. The SAE stated shearing force of a 1-3/8" fine thread fastener is around 1200 ft-lbs, so no worries about stripping a usable axle thread with a cleaned and lightly lubed nut. Note those nuts are supposed to be one-use-only, but I know the price of new nuts is high ($16 ea?) So we all reuse them. Mostly, this works.
There are two different axle nut sockets: the OEM factory tool is a face-pin spanner with four square protrusions. The late model socket now sold by the vendors is a socket with 4 protrusions inside so the socket completely encloses the nut. This is a much stronger, more slip-proof design, well worth the price. There are also two different axle nuts: one is fully threaded with 4 full height notches; the other has cut-down sides about half way resulting in half-height notches. The Factory face spanner socket will not reach far enough down to fully engage the cut-down nut's notches and may deform or strip the notches at 400 ft-lbs torque.
One last warning: if a previous owner was a little paranoid and used red Loc-tite 271 AND torqued the assembly to 400+ ft-lbs, it may be better to blow the nut off with a cutting torch or use a Dremel grinder rather than hurt yourself- or the assembly- trying to loosen the nut. If the nut simply won't move and you're turning the nut in the correct direction (dextro is right-hand-thread and sinistro is left hand-thread; the marked axles may have swapped sides), try a little heat: hot Loc-tite has an acrid odor instantly detected by your nose. If you smell something 'sharp' while heating, I suggest cutting the nut off. I have several such 'keepsakes'.