I will say one thing, if you can use an MTX75 box then do it if you plan on some decent power, rather than the 'easier' Fiests ST IB5+ gearbox.

The IB5 is not really that strong, plenty of peopled had them fail at very moderate power levels...replacing the diff does make them stronger (as well as bring the obvious benefits an LSD brings) but if you have a choice, stick with/go for MTX every day of the week