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The threads are stripped where the drain cock goes into the radiator. I tried running the threads with a tap with no luck.
is a helicoil the way to go with this?
Or is there a better way to fix it?

It's probably a pipe thread (NPT) which is a tapered thread, I'm not aware of helicoil repairs for that type of thread, though someone else may have other information.

You could try soldering the petcock into the tank lug. Or, if there's enough "meat" in the metal lug, it might be possible to drill and tap for the next largest pipe thread and then use a threaded NPT bushing to reduce that back down to the drain cock thread. Otherwise I suspect you'll need to have the lower radiator tank removed and repaired.

I don't know what year and model Packard you're inquiring about but if it's not a pressure cooling system, there may be some other workable though less than ideal solutions, for example an epoxy rated for that kind of service, perhaps J-B Weld? I'd feel a lot more comfortable with solder than epoxy.

Thanks hoping to not pull the radiator but if I have to.

Dear Appin:

On a lift with a good set of torches and a skilled person a new brass one can be soldered. The metal has to be cleaned meticulously, then tinned. Laquer thinner will take the paint off, PPG DX440 will get the grease and oil off, followed by sandpaper and wire brush, then more DX440, then distilled water, then a good liquid flux rated for brass and steel.

Sound expensive? Well not really if you're repairing a Senior radiator in the car.

Once the new unit is installed do not tighten more than enough to close the valve. Just like a dripless bathroom sink. It will stay closed because particulates in the coolant act like thread locker.