[Ruscal]

Going with a vendors 2", 3" drop shocks is a general guide to fitment, but to do it more accurately is a lot more hassle, but takes some guesswork out of the job. Following is how I did my front shock install.
First install the control arms and bump stops you will use. Then install the springs to get your stance just right. You can then take an eye to eye measurement with the vehicle on the ground. With that measurement in mind you can check shocks available from vendors. Keep in mind that you can relocate your upper shock mount to accommodate a longer shock if needed. A longer shock typically yields more usable travel.
Next you need to remove the spring and collapse the suspension onto the bump stop.
Take another eye to eye measurement. That measurement then has to be shortened a guessed amount to compensate for bump stop compression. I used 1" for my Hotchkis beehive bumps. Record that dimension.
You can now calculate the travel you will need, which will give you an idea of the total shock length to buy. Don't forget to figure upward travel for when the truck will become unweighted.
I thought it was best to relocate my upper shock mounts upward a few inches to fit a shock with plenty of travel. I also moved the mounts forward to get better alignment with the Hotchkis A arms.
I used Ridetech's catalog to select the closest shock to my measurements. They have a good selection of sizes.
Once the shocks arrived I then mocked up the upper mounts to optomize function. You want the suspension to bottom before the shock does. I have about an extra inch clearance. The mounts ended up about 3" higher than stock. I am using Hotchkis springs that probably drop about 2" Plus or minus. I purchased Porterbuilt upper mounts, but thought I could get better positioning by fabbing my own.
I have some pics on another computer I will include in a following post.
The rear shocks can be calculated in much the same way.
Russ