The Pontiac Commercial Chassis used special heavy duty coil springs, and I doubt that you will be able to find anything that closely resembles the spring rate of the originals. Spring rates are determined by the thickness of the wire, and the number of turns in the wire of the spring. GM used different spring rates on each side of the car depending on the equipment that was factory installed, and the type of suspension that was ordered. Although the springs all look the same, when you take the measurements, that is where the difference is. I remember seeing an old movie that was made by Chrysler Corporation showing how springs were made and calculated. An identical spring in physical characteristics, might have as many as 10 different spring rates. Each spring was put into a measuring device, and checked. Then it was put into separate containers depending on its spring rate. They would put different color paint marks on the springs to differentiate between each rating. The spring rate along with the type of shocks determines the ride quality and ride hight of the vehicle.
I went through this very same discussion with another PCS Member a few years ago, but it was after he had replaced all the springs, and the car was actually worse riding with the new springs. He eventually put back the originals, and installed coil over shocks in the rear, and heavy duty gas shocks in the front. That worked out well in the end, and it was also a 1967 Pontiac Commercial Chassis car. If my access on the other site wasn't severely restricted, I could locate the original post for you to read.