ECU box ground.

It is a waste of time...& cable. Mopar inner fenders are spot welded to the f/wall, sub chassis. We are talking about 6 amps [ max], not 300-500 for the starter. There is enough 'connection' there for a more than adequate grd through the welds. Make sure the metal is clean on the box & mating surface on the fender, use a star washer between box & fender, job done.
If you were to extend this separate grd cable to the battery neg logic , then you would have grd cables from the h/lights [ about 15 amp for 4 hl car ] & one from the cigar lighter, p/windows etc. You DO need to make sure that the grd cable from engine to the body is of adequate CSA & has clean connections at each end.