First of all I learned that the spring packs changed mid-generation. I
think the 68 and older trucks had a different number of springs.
I do not have experience with their lengths or the individual spring rates of each leaf(meaning that they simply threw a couple extra leafs on for 1 tons; or if the entire spring pack was a completely different design)
I believe the extra leaf pack you may have could be the auxiliary springs on the earlier trucks, someone deleted the aux spring from my '72 1 ton SRW and according to a website my aux would have only been a single spring.
Aux springs were not standard, they were optional.
This site is what kinda confirmed that.
https://www.sdtrucksprings.com/leaf-...c/pickup-67-72