It is a very easy mod. I did my brothers 73 Charger last summer. Summit Racing sells a kit for about $20, you get 4 or 5 .019 shims and a metal sleeve to hold the valve in a vise to take it apart to add the shim to. Or you can make your own shims.
Steering box was a stock, no feel box. IIRC the original valve had .042 of shims in it. We added 4 of the .019 shims for a total of .118.
To get to the valve, remove the 7/8" nut from the backside of the pump housing, spring and valve reside down in there.
Steering definitely had a much better feel.
I'm sure if you had a FF or Steer n Gear Stage 2, you could add shims to tweak the feel if you wanted it a little stiffer.