So, the SPN at Newark will include Valiant (50,377), Dart (55,287), Fury (39,134) and Monaco (20,166) - 100001 to 264964
Those model totals for Newark were one of the things I was looking for! Thank you!
SPN for Belvidere includes Fury (91,368), Monaco (58,158) and Chrysler (8,194) - 100001 to 257720
Eliminating DH46T4D258009 because unreliable I actually found no 1974 SPNs that exceed the total production per plant. The 8,194 Chryslers at Belvidere already easily make up for the SPNs I thought were missing.
Therefore it doesnt't matter that Newark also built 1974 Valiants and Darts. Their SPNs are not intermingled with Fury and Monaco SPNs.
Total production for 1974 - Fury (130,502), Monaco (78,324), Chrysler (117,373), Imperial (14,426)
Interesting that your data agree with a total 1974 Monaco production of 78,324. That means that fleet Monacos are already accounted for, although their exact number remains unknown. As a matter of fact the two SPNs I found for police cars, DK41U4D215212 and DK41U4D215750, stay well within the 257720 Belvidere margin.