What I figure is that if I know what the stock resting length of the shock is then I could manipulate where I mount the upper to get it more where it is designed to be. Does that make sense?
Yes, but you still need to be sure that your ride height and mount location is within the working range of the shock absorber.
Otto's truck has stock suspension so his ride height is likely very different that yours.
Here's a
link to measurements for Monroe shocks. What you don't want is for the compressed length to be less than the movement of your suspension at full compression. Measure from the axle to the bump stop on the frame to get that minimum length. Max extension is with the axle hanging free, plus a little more for overshoot if you catch air. But that's not as big an issue as over-compressing the shock because there's only the weight of the suspension pulling against the shock. On full compression, you'd be applying the dynamic weight of the truck chassis against the fully compressed shock if the bump stop isn't reached. That would likely damage the mounts or the shock.
