All 1971 340 e-body cars came standard with the S74 quick ratio steering box as of February 1, 1971.
Not really sure if was available as a separate option for 1971.
If your car was built after the above date, it should already have the fast ratio box installed unless it was changed out in the past. this was true for both 1971 Challengers and Cudas
Another thing to remember; most of the difference in quicker steering was due to the longer steering arm and special idler arm. The difference in the box itself was where the "stops' were set when going to extreme left and right turns. The stops were set shorter because the bigger 15 in tires would rub the fenders at extreme turns so the stops were set a little tighter to prevent this from happening.
The power steering pumps are different and cannot be interchanged because a higher pressure ps pump was installed (as per the bulletins)