I currently don't have control arms in the car. I need to break out the old upper ball joints from the different control arm I'm putting in.

I've been advised to not just hit the ball joint with the impact wrench, as much as I want to.

Should I bolt them back into the car once I press the two UCA bushings back in and use a breaker bar? Will heating them change things for the worse?

I'm looking for the means that will be the least likely to destroy these.

If your inpact wrench has enough "oomph" torque, to get the old ball joints out, go for it.
It's not going to do anything to the UCA.
Mechanics have been getting them out with air guns forever.
That's what i did, when i was professionally wrenching, for a living.

I've got an Electric Milwaukee that has never ceased to amaze me.

Air wrench it out and be prepared to tack weld it afterwards to be safe

I'd be concerned about tearing or galling the threads. Why make more work for yourself if you can avoid it? Run a piece of pipe through it, bolt that to something, and do it by hand. If it gives you any grief, you can try tightening it ever so slightly to help break it loose. It's one thing doing it on the clock, quite another doing it at your convenience at home.

Never had a problem using a 3/4" drive ratchet and the proper socket to get them out and back in, while still on the car though. I also don;t work on rusty crap either so that probably helps.

Wouldn't hurt to soak in PB Blaster overnight, either.

It's not going to make any difference if you take out a ball joint with an air gun, or a 3/4 drive breaker bar.
If they are tight, or rusted to begin with and are hard to get out, and tend to strip, you still gotta get them out, no matter what method you use.
Now cmon, your not working on the space shuttle here, just a 40+ year old car.

Sometimes this car amazes me how much it fights. I did the "drill a hole in the frame trick" to spray PB Blaster at the k frame bolts for week before I tried pulling the k member. First pull with the cheater pipe instantly sheared the head off.

This is why I ask questions lol.

I'm going to stick them in the ultrasonic cleaner at work with some evaporust to get the rust out of the threads. That should take out most of the battle.

That's exactly what I'm talking about. If you enjoy shooting yourself in the foot, a willy-nilly application of power tools is about the biggest gun you can have. Slow and careful is how you work on large rusty fasteners if you care about trying to save them. It's a lot easier, and cheaper, to keep what you have than tracking down replacement parts.

Hopefully it will. I once had a donor car shear off the captive nut inside the frame rail when I went to take the K frame bolts out. I had to pry down with a pry bar as I turned the bolt to get it out. Pry bar slipped, nailed me in the temple and I ended up in the Emergency room getting stitches. Wife thought I took my eye out due to the blood in it. That was a car from the coast and it was the last rusty POS I ever turned a wrench on.

These aren't the original control arms to the car. I put the tubulars on and sold my factory drum sprindles and control arms. Dumb move.

Years later, I bought B Body drum spindles for the extra viper C-body brake kit I've got and I ended up buying nasty dirty upper control arms from Ohio of all places. Normally I'm not plagued with stupid issues, unless I cause them.

If you are dealing with loose, nasty, dirty, and likely rusty upper control arms, and you are using a 3/4' drive breaker bar, this is a possible scenario:

a. UCA is in a vise. Ball joint won't budge.
b. Add a cheater pipe on the breaker bar. The ball joint still won't budge, but now the table that the vise is bolted to is moving.
c. Add a longer cheater pipe and anchor the table to the garage framing. Ball joint still won't move, but now the vise is bending the control arm.

Lesson:
Get an impact wrench on those ball joints to save damaging the control arms.

In my case, the UCAs were still on the car. But the forces needed to loosen the ball joints with a breaker bar were more than enough to push the car off the jack stands. An impact wrench was the solution.

Often the "shock" of an impact works better than a breaker bar IMO.

That and I guess most dont know that new ones cut threads as they go in.A good impack will get most out.Never had a problem and I started doing them in the 60s.Rocky

Unless it is important to you.
I just purchased two new upper control arms for my 70 A66 Challenger from Rock Auto for 142 each. They have new ball joints, and new control arm bushings. I weighed the labor and time involved with cleanup and removal and went with new, stock looking UCA's that are made by Goodmark.
Just a thought. It would be a lot less work and the advantage of new parts.

I've got about $80 in both control arms at the moment. If they give me any problems, I'll order the firm feel control arms.

After installing tubular upper arms with the added caster, I will NEVER go back to the outdated, stamped steel arms. Unless, it's an absolute matching numbers car.

After fighting a good battle they're back in.

I was able to get the ball joints impacted out with my Milwaukee electric impact. I broke them loose with it and followed it out with a ratchet. The threads one one look like they took a beating, but they still work. It was full of old grease that had dried out and become brittle. I threaded the ball joints into each again and they snugged up nicely.

Should I consider putting a few small tack welds on each ball joint? Or do the threads usually work out?

Before you try any method mentioned. Clean the area very well, and inspect the Ball Joint for tack welds. Not doing this is what caused me to twist my UCA into a pretzel.