Do you mean add a leaf to the springs you have or replace the 6 leaf stack with 7?
Either way it will stiffen the ride up some. Only you can be the judge of what is too stiff tho.
I've added a leaf before, basically cut the eyes off 2 main leaves and put them under the mains. It took the sag out of it and at the time I didn't think it rode bad but that was decades ago and ride quality was secondary to getting the bumper back up where it belonged.
Did you call John R Spring. They are a manufacturer in the Detroit area.
John R. Spring Service, Inc. | Spring Manufacturer
Kevin