many years ago I converted a three bay to a four bay. If you have space on the rack, coleman can sell you a pre built pump assembly on mounting plate that will bolt into place on your frame. It's not the cheapest way, but everything will match. Another option is find someone that has converted a SS bay to an auto and has an extra one lying around. I have one, but have been meaning to convert it to a washdown pump for the longest, so I'm hanging onto mine. You also have to add the appropriate wiring and low pressure solenoids,