[hatzie]

You've got Gremlins... but also a good starting point to troubleshoot.

You're doing voltage drop tests and you exposed a possible problem.

You've exposed a possible ground cable issue at the engine end... You should've seen 12.38 or 12.39v from the Alternator to the battery. You got 12.24v... a .18v drop doesn't indicate a good stout connection from the ground cable to the alternator bracket. At most you should see a .03v drop like you see at the starter... .18v is 6x .03v on a much shorter cable than the one at the starter. That ain't good.

Remove the ground cable and clean up the connection to the alternator bracket. You may also want to remove all the alternator mounting bolts, clean em up, and add some Never-Seeze to the bolts before you snug em all back down.

From your test at the starter... the block seems to be getting a seemingly reasonable ground... but that can be misleading... You may be grounding from the smaller gauge body ground on the rad support, around the fenders and, through the cab ground strap to the LH head. If that's the case it'll disappear when you actually ask it to pass enough current to turn the starter.

The next place to test if the ground doesn't fix it...
Do you get at least 12.38v from the J-Studs on the firewall to the block after you twist the key to the CRANK position, killing everything, and leave it in the ON position with everything dead?