How to fix steering slop?

The next step is a new or rebuilt steering box.
The Mopar unit is notorious for wearing out faster than other models. I had Chevy steering boxes that felt great despite being over 100,000 miles. Saginaw built the units for GM.
Ma Mopar built theirs in house. The design you see in the last rear wheel drive cars in 1989 had a steering box design dating back to 1957. Yeah...a LOT of things change in 32 years but Ma Mopar stayed with the same old design with very few changes in that time.
You can have yours rebuilt but due to it's design flaws, that slop will return.
Lares makes a new version but it is anyone's guess as to whether it will develop slop at the same rate as a factory one.
Borgeson makes a unit that is a newer design and it is far better than any Mopar unit. It isn't cheap. You're looking at $1000 for a kit that bolts in with no modifications. I've done the swap in 2 different cars and yes, they are great.
Your other option is to just replace yours with a low mile used one, preferably from a light A body that had a slant six so it has the least stress and wear on it. You will still likely notice some slop and it will get worse over time.
It all goes back to the flawed original design of the 50s. Back then, effortless power steering was thought of as a luxury compared to the hard to handle manual steering the cars and trucks had back then.
Good luck.