You can fit any i6 e series box into any other e series, WC or not.

In terms of strength they are very similar, but the WC box in the long run is superior for a daily driven car expected to last km's with better synchros, needle bearings under gears (as oppose to journals) and tapered countershaft bearings.

Of course if you put it behind a blown/worked engine and drive hard both boxes will break well before their time. The only answer to this is straight cut gears & an aftermarket case, by which time you are into tko/t56 territory...playing with a t5 really is a roll of the dice and depends how you lay down the power; I have seen a 220ish rwkw car destroy 3rd in one run down the quarter, yet some get lucky with MUCH more power. In the end traction + 3rd + torque is always the weakest link.