It depends on which spring rate you choose. And for a MUCH softer ride run 20 psi in your tires like I have been doing for the past 20 years. Because of product liability lawsuits G.M. was forced to recommend higher pressures than needed but the 20 psi recommended for the 1974 C3's is the ideal pressure. If 20 psi seems absurd to you I'll further explain why the later C3's recommended higher pressures:
Back in the days when we averaged 15,000 miles per year and our dealers changed our oil every 3000 miles our dealer checked our tire pressures every 3 months. Okay so far but when the oil change intervals got changed to 5000 miles about 5 months went by between tire checking which meant a slow leak could result in a dangerously low pressure. Then when the oil change intervals got changed to 7500 miles about 7 months went by between tire checking so a slow leak could result in an almost FLAT tire by the time our dealer checked the pressures. Then came the 10,000 mile and 12,500 mile oil changes; meaning 10-11 months went by. And with our present 15,000 mile oil change intervals a whole YEAR can go by between tire checking. See the problem extended oil changes caused over the years? But for those of us who check our pressures every month or so 20 psi is the perfect pressure to run to get the maximum tread life out of our tires. My front tires now have 50,000 miles on them and 1/3rd of the tread is still left. And with 20 psi the ride is as soft as a full-sized passenger car that uses the same tires but weigh 1000+ more pounds.
Both of my '72's, one a small block car and the other a big block, have a Vette Brakes 360 lb fiberglass installed. These replaced the standard 9 leaf spring on both cars. On the Lt-1 vert, the new rear spring was paired with the 550 lb springs up front. The spring change made the car ride a bit stiffer, but not, in my opinion, harsh. On the BB coupe, the new rear spring was paired with the stock front springs. Spring change on it didn't change the ride much that I could notice. It's hard to explain this, but with the fiberglass spring, the ride seems to be smoother. I discussed this with Corvette buddy who has a fiberglass rear spring on his car and his opinion was that with the standard 9 leaf spring, there is a certain amount of spring bind from the leafs rubbing against each other that you don't have with the monoleaf fiberglass spring. It's also a quiter ride, with fewer squeaks in the rear suspension. Bottom line is I don't regret changing the rear spring out in both cars. Another thing that affects ride quality is the shocks. A stiff performance shock is going to firm up the ride over a standard shock.
I would never advocate running less air pressure than what the car manufactor or tire manufactor recommends.
For a given spring rate, a plastic spring will make the ride somewhat less stiff. That's because when you replace a 7- or 9-leaf spring with a single leaf composite spring, you eliminated the damping inherant with the friction between the leaves.
That said, I also need to mention that "harshness" in as devined by suspension engineers has less to do with spring rates than it does bushings, dampers and tires–the things that affect the high-frequency ride movements some call "harshness. Spring rates have a more noticable effice on lower frequency ride movements.
If your car rides harshly it could be caused by polyurethane bushings if you have any, shocks with agressive valving and tires at too high a pressure.
Most car owners rely on their dealership to keep their tires properly inflated because the average car owner doesn't have an air compressor nor does most 'Quick Shop" style of gas stations. So when the 3000 mile "full service" oil changes vanished in the middle 70's tires didn't get checked for longer and longer periods of time. Whereas the usual 3 month oil changes used to be sufficient at maintaining proper pressures the yearly oil changes of today allow tires to go virtually flat by the time the next oil change is due and that's why the vehicle manufacturers now recommend maximum pressures to prevent them from being drawn into product liability lawsuits.
Drop your pressures to the 20 psi recommended in the 1974 C3's and you'll find your tread life will increase 100% and the quality of your ride will also increase 100%.
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