351w no power

I changed the heads, head gaskets, fixed the timing and a new water pump, starter, battery, and starter solenoid. I’ve already had it running for a couple weeks now and just recently it’s been giving me starting issues. It would crank over super super slow, and all of a sudden it just stopped trying to crank all together. It doesn’t even light up the lights in the car. The battery was tested and it’s still good, new battery terminals and good ground. Does anyone have any idea about what part went bad?

or you can check for voltage before & after the fusible link, 

poke the sharp pointy probe into the wire insulation to contact the conductor inside

Check the battery posts with a voltmeter to test that. If it isn't over 12v, charge it soon, before attempting to crank the engine again. Checking the voltage at different places will tell you where problems are. If the battery terminals are bad or corroded, then the voltage will be fine at the posts, but not on the terminals or cable ends. You can narrow down where the problems are, just go at it systematically. Begin with the battery, the posts not terminals, then the other main contact points. Corrosion can be inside a wire, a cable, and it has to be replaced.

So note what the voltage is at the posts, the terminals, the starter solenoid contacts, with and without the engine running.