You certainly don't want to get rid of the car over a charging problem. You have already replaced the only two expensive parts of the charging system, hopefully not completely unnecessarily.

You say that you have an ammeter that you used to check for drain when the car was off. I presume that this was an in-line ammeter that you put between the battery terminal and its clamp. Should have seen around a 10-30 mA drain with the car shut off, right? That would be normal for most cars.

That means that the car is not charging, or the alternator is not big enough to keep up with your accessories.

You sound knowledgeable enough to realize that if you have an 80 Amp alternator and a 100 Amp amplifier blasting the tunes, the math is not going to work. The alternator will not keep up, the battery will drain, and the alternator will overheat trying to keep up.

With the car running at fast idle, get some voltage readings. Positive battery clamp to engine ground, hot alternator post to engine ground, alternator case to engine ground (that one MUST be 0 volts). Examine the wires that attach to your alternator. If it is supposed to have a ground strap (and I think your's is), make sure it is clean and tight. I think on your car there is a wire that goes from the alternator to the charge light in the dash. If you can, check for continuity in that wire (a bit of a chore).

Your biggest challenge is that you could use a good wiring diagram. I have had up to three manuals at a time and found that the manuals did not agree on the wiring for the same year 240, and none of them was correct. I found that our '87 240 ignition came closest to the ignition that one manual showed for a 940. We got that car new and it was dealer-serviced, so I am pretty sure it was not modified.