Can anyone 'splain how to measure "Arch"??

[Desert Dan]

Generally it would be better to build a whole new spring pack made up multiple leaves than just adding an add-a leaf.

The ride would be better and the spring rate more progressive

You could also add a second stock leaf under the main leaf to give a little more load capacity. Ive done that on Jeeps and Toyotas.

One down side about springs under the axles is that the more leaves you have, the lower the u bolt spring plate and skid are as far as ground clearance