You might have some luck measuring arch and other dimensions, then comparing with the Dodge and Plymouth spring specs at www.Stenglebros.net or .com
You can estimate spring rate by measuring the distance moved under a known mass. A worn spring will have lower spring rate than when new. I had a set (aftermarket) that went from 180 bs/in to 130 lbs/in on the right side, not so bad on the left. However that was 80,000 miles of use and abuse including drag strip and autocross...
Chrysler part numbers for the springs and spring packs (the assembly of 4 - 7 leafs) from the 60s start with a 2 or a 3. However I do not know if the springs themselves were stamped with the part number. If not, it would not be the only Chyrsler part that had different numbers on them than their part number. Do a websearch here and on moparts for leaf spring numbers and you may turn up some more insight on that from concours restoration people.
My notes from Chrysler Parts books have include these A body 5 leaf spring pack part numbers
1967 for 225 equiped B3004 575 and 6
1968 for 225 equipped 3004 581
1969
These for 225 Barracuda HD and Fastback may or may not have been 5 leaf
3004 582
3004 585
Sorry that's all I have notes on, and I can't be 100% sure of the accuracy or relationship between the parts book (replacement springs) and the factory installed. I've read instances where replacement parts were not the same as factory installed....