Which gas tank is main tank?

[bodyman922]

My 78 GMC needs a new gas tank sending unit. When I look on rock auto's website they have a few different brands to choose. Most list if they are for drivers or passenger side except for the dorman ones, they list for either main or auxiliary tank. My questions are which tank is main and auxiliary tanks? Is there a certain brand of sending unit that is preferred; spectra premium, dorman, or other? Thanks for the help.
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