Approx. 1 volt drop between battery and fuse box

back years ago when I could afford to restore a car and do a nice job, after dropping that fresh engine back in ( back then a B body), I always bought a repop engine wiring harness from the guy and his wife that made them, ( they always had an ad in MCG)... I forget their name, maybe still do..... main reason I did was the original harness was usually somewhere hacked up, the plastic covering the wire was hard, cracked and broke open in many spots. plus it made a restored car look " finished"..
I ask this question from you guys that are auto electrical knowledable, ( example: 67 Dart 273 for instance.), is that plastic covering that copper wire having those cracked places, does that really pose a problem with current flow? I realize most of the electrical problems we find is probably from the bulkhead connection to the interior.
I admit, i havn't bought a new harness since maybe 2003 or so. I get my herd running and driving with the old harness, and yes, seems like all my electrical grimlins always seem to stem from the bulkhead inside.