There is a bolt in the original steering knuckle/spindle that appears welded in. I'm guessing it controls the amount of turning radius that the knuckle allows for. Both of my new knuckles do not have this bolt but there is a threaded hole.

What is the purpose of that bolt? Should I add a bolt to the new knuckles?

Yes it is to limit turning so the tire doesn't rub the lower control arm .

Any idea what that bolt is called?

Looks like it is this part: https://www.moparpartsgiant.com/part...~s0114494.html

It's just a bolt and jam nut threaded into the knuckle. The nut is typically tack welded to the bolt, but you can unscrew the whole thing from the knuckle. Sometimes you just need to add a washer underneath the nut if your bigger tires are running.

The new knuckles I bought didn't come with the bolt. They take 3/8"-24. I used 1 1/2" with a nut at the base like the originals had. I measured the old length and screwed in the new bolts to that length. When I'm sure that's a good length, I'm going to loctite them in.