[franken]

Assuming heat soak (one of the less likely issues) for no particular reason is a mistake. Basically, it means you have a problem and decided the cause w/o a reason. The same goes for any issue.

For the starter (slow crank), the ground should go from the battery to the block. The + cable obviously goes from the battery to the solenoid. Good battery cables are needed (good and new may not be the same thing).
You can use one lead of a jumper cable to test the ground or + side.
You really need to know what the battery voltage is at rest, under load and when the alt is charging it. That means you need a decent voltmeter (DVM). A charged battery is 12.6V, Charging 13.5V or a bit more. You might also measure end to end from each battery cable terminal on the same cable.
As people stated long ago, initial timing too advanced, will cause a slow crank.

I read the whole thread and don't think I saw any voltage measurements. You can swap parts for years (as you have) and not solve the problem. Maybe it's time to troubleshoot the issue... Sorry if that's pushy but it's still a fact.