Anything more than 300 ft/lbs and you're on borrowed time especially if you drive like a loon, you have to watch the big boost spikes particularly with 3rd gear, as said people do run big power cars with standard mt75 boxes, I ran mine for a few years with 400ft/lbs and the box lived even to be sold on when I had finished with it so driving style has a lot to do with it

Luciano