I have a 1987 560 SL that had the same steering box looseness. The adjustment process is fairly straightforward, the access is difficult. You will need a crows foot wrench on the locknut, and a long allen head for the adjustment nut. As noted above, I erred on the side of leaving a little play in the box. Big improvement overall.
1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.