1966 Ford F100 won't start without a jump?

Battery is only a few months old and holds a charge when i pull it off a trickle charge. I had the alternator tested at o'reilly auto and it also passed on their machine. I replaced some very corroded wires and when the charging system is fully assembled it holds at about 12.2 volts. It being an older system i'm not sure if that is low or if it is spec. I would like to know before i spend the money on a new alternator and have it continue not working. Outside of that i replaced the wires to the battery terminals and from the starter solenoid to the starter for corrosion as well, and the truck runs without issue after the jumper cables are disconnected. Any direction helps.
Thanks!

Added (1). I will have the battery tested today. But doesn't an alternator usually put out 14V? The battery is holding the charge that is being put into it. Even with the engine on it only puts out 12.2V. Thanks again

Added (2). It turned out to be the alternator, after it was replaced the charging system did start putting out 14V again. Although in the process it was running off of the battery but i was able to exchange that with the warranty at no cost. The truck is back on the road again.

The alternator isn't working. The 12.2 volts is from the battery. Taking it out to be tested is worthless. The only test that matters is in the vehicle with the engine running. The truck only runs after you pull the jumper cables because you've charged the battery a little.
Troubleshooting it depends on if it's the original generator and external regulator or someone converted it to a newer alternator. You might need 12 volts to a field terminal for the generator or some alternators to run.

12.2 is a discharged battery

I agree have your battery tested s/b 12.5-13.1 fully charged.

With the engine running the voltage across the battery terminals should read at least 14.2 volts.

It will be a dynamo.
Totally different beast.
You need a classic car electrician not a parts place.

You need to put a long, slow 5 or 10 amp charge on your battery. A fully charged battery should be at least 12.6 volts. The alternator should be charging at 14.4 volts. It is not the alternators job to charge the battery, its job is to MAINTAIN the charge. Running with a discharged battery is about the best way to overwork your alternator and kill (which, at best, stock on this truck 60 amps, probably lower).

You need to have 13.7 volts at the battery posts with the motor running and at 1500 rpm or the alternator is not working. The fusible link often fries and needs to be replaced. The alternator will ot charge the battery if the fusible link is bad. That old a vehicle may have fusible link wires at the battery positive terminal. Newer vehicles have larger fuses in the engine compartment fuse box which are also called fusible links.

That old, the charging device would be a generator, not an alternator. But regardless, if it is charging, a voltage test on the battery with the engine running should show in excess of 12V., especially if you turn on the headlights. A generator may require the engine to be revved a bit for a test.
As well, if checked at night, you may see the headlights get brighter when you rev the engine a bit. That indicates charging.
It is not impossible to change the existing generator system to an alternator from a later model vehicle, one that has all its regulation inbuilt within it. Alternators are a far superior charging unit. The only problem is some engineering ingenuity may be required to fit the alternator mechanically.
If the engine starts with a boost, it tends to indicate the wiring is fine, but if you are not boosting directly to the battery negative terminal you may have missed a bad cable (possibly the earthy one, or a cable from the chassis to the engine block?), or maybe the battery terminals are not contacting the battery posts properly? Are both clean and tight?

Engine idling with headlights on, the battery voltage should be around 14.4 volts.

Your truck has an external, mechanical voltage regulator. You have old wires and connectors. There may be corrosion. You may have rusty sheet metal. The voltage regulator needs to be grounded.

The "generator" light is part of the charging system. Does it work? When you turn the ignition on, before you start the engine, is the "generator" light on? Do you have an amp gauge in the instrument panel?

You really need to understand how the charging system works, before you can diagnose a problem.

Modern vehicles have an electronic voltage regulator inside the alternator. The whole charging system is in the alternator. Yours is not.

Charge your battery for 24 hours with a 10 amp battery charger, not a trickle charger.

p.s. You can buy a new battery, then leave the headlights on, and within a day or so, the battery will be dead. And "it's only 2 days old" doesn't mean much, does it?