WHBM, the 787 interior is 2.5 ft. wider than a 767.
However you are correct, Boeing designed the 787 for 8 abreast in economy - in much the same way the 777 was designed for 9 abreast. But as everyone else has noted, seating configuration is up to the operator, so long as it doesn't exceed the max rated count (which is based on the evacuation capability, not anything to do with comfort).