Motor ID

Don't put too much stock in the casting dates. They mean exactly that, the date the casting was made. Unlike some other manufacturers, MaMopar let castings sit around for 6 months before they were machined for assembly so they could fully cool and take a set. The assembly dates were stamped in the finished products. In the case you are citing, the SS Darts had Race Hemis instead of the street version, so it is very possible that some '66 castings were still in the race shop for assembly as a '68 model.