Clutch fork won't clip back in.

May be wrong if I am not following your question, but no need to move box back, just lift spring clips back enough with fork ends so as to have fork end pinched between throwout spring clips and throwout bearing main fork thrust body surface. Here are a couple of front and back pics of it installed correctly. The fork will be allowed to slide into the locating tab on the bell housing when it moved further forward in-between the clips on the T/O bearing, somewhat of a simultaneous insert.