It also comes down to how you drive the car, as well as the amount of power you. If your rough on it you can destroy a "fully" built box with just a mild motor, on the other hand big power cars and cars with forced induction are out there that are powering.through relatively budget built boxes.
If you are chasing big power in the future you have a few options if you want to keep fourspeed and conveetor lockup.
Either get your box "built" by a reputable auto specialist that is going to fill your box with what he says he is or, you can upgrade to a built 4L80E which is.going to require tunnel mods, and maybe crossmember and tailshaft mods as well.
Or you can buy an expensive, built 4l60e outta the states, there are blokes over there that have built boxes with hardened.and oversized input shafts, which is the weakest link in the box.