OK. According to the Chrysler Muscle Car Parts Interchange Manual, there are two different steering boxes for 1968: early and late. Apparently, the outside-diameter covers are of different sizes. The 1974 Dart box has a completely different interchange number. Looks like the Dart box was used in the 73-74 E bodies, charger, coronet, and satellite.
The only interchanges shown in the book are among the 68-69 Baracuda, Belvedere, Charger, Coronet, Dart, and Valiant.
As Psycho pointed out, the differences may but relatively minor.
Hope this helps.
