I run two batteries, for years we've had starting issues slow, lazy to turn over sometimes barely catching. So I put two batteries in there to help. still the same issues. I ran ground cables from the battery to the block still the same. I put higher cranking capacity batteries in there, still the same. Motor would spin ok when the batteries where charged so I put a bigger alternator in there, still the same. OK I thought thats just the way it is. Since I run an MSD grid, electric waterpump, fuelpump, fans, etc.I put up with it and made sure the batteries are charged before every run. I put a small logger system on it and the voltage would drop progressively to 12.5V by the end of a run. I also had a cheap battery charger that went faulty, it would damage the batteries when connected (the output wave form was incredibly coarse with a 6 - 20 V wave as seen on a 'scope)
I found out what the main problem was only a few months ago, I've always run a narrow alternator belt so it's sits low in the pulley. It was so narrow it was running on the bottom of the groove and slipping when hot, not charging. All the symptoms made sense, don't overlook the simple stuff as is my way sometimes I over complicated the problem.
The wiring is also a mess, not dangerous but unnecessary lengths / runs / borrowing lives done over the years as I've added stuff to the car. So a total re-wire is on the list for this winter, I'll be using some of the tips I've learned from here regarding wiring and grounds etc for sure