Huge voltage drop across battery while running

I would go clear back to step 1 so to speak the very very basics. You may have a major connection problem, or you may be missing something

You say you disconnected the alternator. How did you do that? If you simply disconnected the output connection, that can damage the alternator, as you must disconnect the field so it does not try to charge. Otherwise, with only the output connected the output V at the stud will go way way up.

Do you have a way of testing the battery, AKA load?

Otherwise, disable the ignition. Measure battery terminal voltage, and make sure it is good and fully charged. 12.6 is a good figure

Now, while monitoring battery voltage FIRST at the posts themselves, and second at the starter and the engine block, crank the engine a few seconds and read the meter while cranking. This is easy if you jumper the start relay. Bare minimum is 10. to 10.5, higher is better.

Not knowing, is your wiring factory? Modified? Generic harness? Anyhow, you need to trace the main path from the battery, usually to the start relay, and on into the vehicle interior to see if there is a bad connection there

What did you do about the ammeter connections? They MUST be bolted together when the dash is out

iF you get nowhere, pull and clean the battery clamps and recheck voltage right at the batt posts and then right at the clamps.

Assuming the starter is cranking OK, what you are telling us does not make sense. The starter and main cables are the largest possible load. For some other problem to cause the battery to fall as you claim is not logical.

MAKE CERTAIN your meter connections are not intermittent, or that you don't have a faulty test lead