If you are running leaf springs,
Disassemble your spring pack and make a mock up pack with some scrap steel so that with the main leaf attached, its the same thickness as the real spring pack. Bolt it to the axle. This will give you a really soft spring rate and allow you to cycle the suspension full up and down, measure the amount of travel you have at the location you want to put the shock and then choose a shock accordingly.
![Beer Chug\'n :beerchug:](https://board.marlincrawler.com/Smileys/marlin/beerchug.gif)