hard cranking after warm

I agree with pulling the coils wire and see what happens.
Old and new starter have the same problem......probably not the starter.
The ground is the best place to start. To make it simple, Get a set of jumper cable and add a ground by putting the Negative(-) jumper from the (-) of the battery and the eng block.
only take a minute and you didn't have to take anything apart.
But my money's on the positive cable' that goes from the battery to the starter. if you have headers, it tends to get cooked at the starter. that one is probably vary old and need replacing anyway.
How ever if you want to test it. Set the voltmeter to 12 volt and tested at the starter and a good ground, is a good test for that. report back how low the voltage drops. then do the same thing right on the battery itself.