Measure the installed height of the springs that are in the truck now. With that info and knowing the rate of the springs you can determine how much weight the springs are supporting.
With that info, contact your spring supplier and tell them you need springs to support "X" amount of weight at what ever height you like. Example: if the springs have a current installed height of 11" and are supporting 400lbs, if you want to lower the truck 1", let the spring shop know you need to support 400lbs at 10" installed height. Keep the uninstalled (free) height the same so the springs won't be loose at full suspension droop. You can make up for any minor differences in height with a small shim or by slightly trimming the spring from the high side.
As to the shock mounts being strong enough, I feel all coil-over mounts should be double shear.