I don't have a book for '65, but based on Stanley Springs carried by Stengle Bros. they were similar to the springs used on the bigger A-bodies '67-9.
http://www.stengelbros.net/Plymouth-Barracuda-Leaf-Springs_c_505.html
But based on the part numbers and descriptions I've assembled from the shop manuals, DC Buletins, and Parts books, they were not the exact same springs.
Here's what I can tell you for sure.
The DC Ralley bulletin says '65-6 Formula S and GTS cars got p/n 2495 086 which had 110 #/in.
The '67 slant 6 equiped Barracuda tested by Car Life had 110 #/in wheel rate and the '67 273-4bbl fastback had 136 #/in. wheel rate.
So my guess is that your original '65 v-8 had something around 100-110 #/in. If it came with a 2bbl, a 4bbl, or a Heavy Duty package might have changed the springs it came with.
The right and left would have had the same spring rate. That's true even if they are constructed differently to account for the extra torque control sometimes needed on the right for certain situations. Not likely on a standard package.
If you test your springs and they have different rates, then the spring is worn out. It's either been cycled beyond its expected lifetime (1 millions cycles is typical design expectation), overloaded or over torqued at some point(s), or both.
PS You can additionally reference the Valiant and Dart springs they offer, for example:
http://www.stengelbros.net/Plymouth-Valiant-Leaf-Springs_c_504.html
If you dig, they even cross reference some of their spring p/n to Chrysler Part numbers.
Also, if you divide the load rate by the free arch, you will get approximate the spring rate.
PSS. Slightly O/T but this comes up so often, allow to add the following. Whether the back of your car sits high, flat or low depends on the weight of your car in combination with the spring's free arch and spring rate.
