On Sunday, July 12, 2020 at 6:13:50 PM UTC-4, wrote:
...I have a voltage lost of aprox 2 volts over the positive wire
and 1.5 over the negative one.

That's a ENORMOUS voltage drop!
I never owned one of these beasts, but...

1) If you let it crank (or try) for a brief period,
the area dropping the voltage will heat up.
First expose as many as possible cables, connection points,
and relays. Then crank for a bit, and feel for hot spots.
You might be able to find problem areas quickly this way...

2) This beast is a kinda old. You've got most likely got bad
connections, maybe starter relay, and possibly cables.
Connection points corrode over time, especially if the beast
has lived in humid climates. Consider undoing, cleaning,
and reattaching ALL connection points.

3) You're absolutely on the right path looking for drops,
and obviously you see there's a horrible problem on both
high and 'ground' sides...

Hope that helps, and let us know what you find!
Best Regards, Dave