Something I've found that works well on old cars I would buy (and not want to invest heavily in them) – hook up a second negative cable. Old electrical cables/wires corrode inside, or sometimes develop breaks in some of their wire strands. Too, the cable or woven grounding strap going from the engines to the firewall were fully exposed, and flexed all of their lives. I'd go to the parts store, grab whatever pre-made negative-lead (in those clear plastic boxes up on the wall) had an eye at one end for a bolt, and whatever connector I needed to attach it to the older-cable's battery post clamp. I'd then run that 'second' negative cable from something like the head, the alternator bracket bolt, or what-not, back to the battery. It just about always made the start motors spin faster and made the vehicles run better.
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