1969 dart alternator problems

so i have the battery in trunk it has o gauge to it and it has about 12.5 volts , at the coil it has 10 . 5 way more than just a half a volt drop . im now having problem with spark plugs running rich or dark looking , im thinking it
has to do with poor spark because i just had a carb built by pro fuel systems and changed plugs and the plugs are carboned up after a couple of days . i also just put a new mh electronic ignition conversion in a month or two ago .should i fix power connection at firewall / bulk head to get more volts to coil