As an engineer, I respectfully disagree. Without appropriate calculations validated by controlled testing, our four cents (your two and my two) are as valuable as...four cents.
Since the data is not at hand, we stray from the "school of engineering" to the "streets of opinioneering". And the streets can be dangerous. Lots of potholes.
If the bottom leaf does not affect ride height, or safety, appreciably, don't you think that Ford would be tempted to use the exact same leaf on every car, and move the part number stamp up to the next spring? Either that or don't you think that Ford would be just a little tempted to leave them out? Especially moving into 1968 as many items were cut for cost?
I've designed components that went into production for Ford and GM. The bottom line is that once you leave the aesthetics arena, there is nothing put in there that was not required from an engineering perspective. It's all about cost vs. performance/safety. I'm not exaggerating - we would literally sit in a conference room reviewing a design and go after the removal of one 10K resistor. I remember the boss saying that "a resistor in bulk cost is $0.72, the axial insertion machine costs $0.12 to install each component, the wave soldering machine...". It all added up to, believe it or not, about 2 cents. It might be hard to believe, but two pennies are sought after with volumes of 500K and up.
With some things, you can go overboard with your safety factor and have little to no consequence. Take an elevated deck on a house. "X" number of 4x4 posts will work just fine. But to be safe you increase "X" by 25% and increase the post size to 6x6. Given the total cost of the job, the small increase in cost is worth it so that you have peace of mind. The design is such that the additional support does not hurt anything or anyone. When you are on the deck with family and friends, people will talk about the nice look, the nice view, etc. But nobody will notice the extra support.
But the thing with springs is that there are some big negatives if you go overboard with your safety factor. Consider the F350 with additional spring capacity to handle a fifth wheel trailer. Then drive down a bumpy road with no load. Not a pleasant ride.
The fact that there are what, 4, 5, or 6 different springs used for each model year (engine size, body style, handling package) should tell us that the rear springs need to be tuned carefully. Otherwise one version would do. And Ford would save a lot.
We all know that most Mustangs sagged. There are many old black and whites and Polaroid's that confirm it. And since this was true when new, the engineers are certainly the ones to blame.
My point was simply this: if you take a new spring, and substitute even the weakest (smallest) leaf with a used one that was loaded for 40+ years, the overall capacity will be lower. Not the same, and not higher. That is common sense.
Will that decreased capacity, of unknown engineering units, result in a noticeable difference? Like a tootsie pop, the world may never know.
Notice that neither in this nor in the previous post did I say that it would be lower or less safe. I'm just pointing out the concern. And I stand by that concern.
