Any leaf spring will stack, just a matter of what the tension screw is set at and the thickness of the spring. The tapered spring is supposed to reduce the stacking some, but all leaf spring guns will stack. A Remington 58 is a cap & ball revolver, and it needs a heavier spring than a cartridge gun. If you have a Pietta, the spring will be heavier than an Uberti. I shoot my "58s" as conversions, and have lightened the mainsprings, but if I want to shoot them as a cap & ball, I will replace with the heavier spring.
One thing to do is to buy a few mainsprings from VTIgunparts.com and thin one out little by little (width, not thickness), keeping track of your dimensions, until it doesn't reliably pop a cap, then thin the spare to the last dimension that is reliable. The failed spring will probably work with a 45 Colt cartridge conversion.
Remove all grinder marks from the sides with files and sandpaper or stones, and don't let it get uncomfortably hot to your fingers, keep dipping the spring in water to cool it.