What the date code is?
According to the Chrysler chart, 2998945 is a radiator part number for 1970 Dart/Valiant, 22", 318 w/o AC or 340 w/ AC.
As said above, 18398 is probably the manufacturer code.
In 1971, this code is 20446. At least, it’s the code of my 71 radiator.