You find any evidence of HD suspension options for any Plymouth of Dodge that year? Not listed in the dealer books at HH. And if the lengths at Stengels webstie are correct, it looks like a one year only spring pack.
For Dodge Darts:
I haven't found any factory information yet but Classic Industries has listings for "standard 5 leaf and HD 6 leaf springs". I'll keep looking for factory information. Lots of options but they are just listed as "HD Option".