Hard start conditions can be caused by something as basic as a single bad connection or as difficult to find as a weak distributor advance spring that doesn't always return the advance back to base timing for starting. Start with the basics- the things that can easily be tested and verified like battery connections (don't forget the ground wire at the block), battery and alternator. This sounds like the original alternator with a mechanical voltage regulator. This is one part that would benefit from a modern replacement rather than a rebuild. Chances are this alternator doesn't perform like it did 28 years ago. Probably the original battery cables too. I am not a fan of throwing parts at a problem, however, if the battery cables are original, I would replace them even if they tested OK. Replacing the ammeter in the dash of my '77 with a voltmeter is on the spring to-do list. They can tell you so much more of what is going on.
One thought...many of the alternators, mine included, need to see a sufficient rpm to get the field excited and producing full voltage. My boat needs to see approx. 1,200 rpm to get her fired up. I ran into a situation where I started the boat up, idled through a long no wake zone and across the lake to shut her down and chill for a while. Upon trying to restart, I barely had enough battery to get her started. Didn't see much happening on the ammeter, but a voltmeter would have shown the low voltage.
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