Most of the detachable mags out their won't have any bigger box that what the factory does so if your looking to be able to extend the length of your rounds you may be looig the wrong direction.
Wyatt's offers an extened mag box for the Win M-70 that says it extrends the mag size from 3.400" to 3.820" , thats almost a quarter inch added length. The Wyatt's boxes are a big upgrade when shooting VLD style bullets but they do require removing some metal for the reciever and that best done by sombody that can operate a mill deciently.