you meen just the hpi diff to fit inside the redcat case? or the whole gear box to fit on the redcat chassis? eather way it wont fit with out major modifacation. the hpi gear box has a lay shaft that is at the top. now as far as "locking" the diffs. that can easly be done with something like silly putty. just packing it into the diff case will make the diff completly locked, and you will be able to easly pull it out "when" you decide its not that great. locking the diffs will create many problems. the diff is already a kind of weak spot aqnd is easly stressed with a large motor/pipe combo. locking the diffs will couse alot of issues with drive shafts, drive cup and diff pins. you will also have very little stearing with a locked diff up front. i noticed stearing loss with just a heavier diff lube on my tt. but try the silly putty in the diffs before making any major modifacations.