Thanks for the replies. I am using the stock tank in the cab. Stevencvn72, your thread is where I saw the picture of the C20 with the two line sending unit. Have you gotten any further with your line replacement?
From what I've seen, a 72 with an auto trans used the 2 line unit and have found a good picture of one at:
http://www.carpartsdiscount.com/auto...html?3593=2633
According to the picture, the return line is a lot shorter than the pickup line and only extends a few inches into the tank. I'm wondering if you can hear the fuel splashing back into the tank if the level is below the bottom of the return. Or if the splashing somehow would cause a problem with the pickup.