What kind of thread to use?

[mountain mama]

Hello everyone, I would consider myself a beginner although I have sewed all my life. I have worked with upholstery for many years, however I  need a little guidance, input or whatever advice you have to offer. I will be redoing the seat on my Kentucky springset saddle. I will be doing a quilting style known as trapunto. This is where you quilt a pattern into the leather with a cloth backing, then poke little holes in the clothe and stuff it with small bits of wool to make raised patterns. You can see examples of this if you look up the Minihan Saddle or Kentucky stitch down saddle. This saddle is my personal saddle that I have road for the past five years. I expect the saddle to have to endure many more years of heavy riding. What type of thread would you use for the quilting? I will either hand quilt it or use my old singer pedal type machine. I know to use beeswax on the thread, I jus don't know what kind of thread would be the best. What type of material would best for the backing? Any thoughts? Please share.