Gas tank sending unit question

[turdmagnet]

I need to replace the sending unit and when looking in the lmc catalog for new ones, I am offered a choice between "3 out" and "4 out". Does anybody here know which one I need? I can't see on top of my gas tank. This is a stock '74 Blazer with a carbureted 350 and a manual transmission. It has a 25 gallon tank on it, but I want to replace that with a new 31 gallon tank.

 

[5280k5]

I'm pretty sure mine has 3 lines but will need to look tomorrow when it's light out. It's got a 1" body lift so top of tank is easily visible
The current tank in it is out of my old '77 and is also the 30 gallon variety

 

[dyeager535]

Fuel in, fuel out, EVAP, and fill neck vent are the four possible "outlets" on a sender. (anyone know I'm wrong? Haven't looked at a carbed sender in awhile)

Obviously fuel out and fill neck vent will always be present. Therefore, if the truck has (or is supposed to have) EVAP and fuel return line, you'd need a 4 out to make/keep them operational. If it has only one of those, 3 out.

That is how I'd approach it.