That tube is the nozzle for both the Accelerator Pump and the Aux. Accel. Pump. It should give a fairly healthy squirt when you work the pump arm by hand (on the front passenger side of the carb).

The most likely place for blockage is right behind that brass tube, where the passage opens up and transitions from nearly vertical to the tube itself. At the top of the vertical passage there is a chamber that the brass tube connects to, and this chamber is enclosed solely by the gasket between the air horn and the bowl (top two pieces of the carb).

There may be a little bit of gunk in there or the gasket may have just gas soaked and swollen over the years, but either way I'd suggest pulling the top part off the carb and having a looksee. You can do this without removing the carb from the truck as long as you are careful.

First pull the two clips between the choke housing and the carb opening. One connects the choke linkage, the other connects the fuel metering plate (thing that looks like a choke on the secondary barrel). Once those are disconnected, you can undo the 5 phillips head screws that hold the top on (one is down the deep hole between the barrels.

Lift the top plate off carefully, since the pivot for the float and the needle valve aren't really held in by anything, and they are all attached to the underside of the top plate.

Once you have the top off and the gasket out of the way, you'll see what I mean about the chamber (small) behind the tube. I'd take the Accel Pump off again and blast the passages out with carb cleaner again until you get some good flow.

Might want to replace the gasket while you're in there too. I don't remember if you can get that one separate from a rebuild kit other than at the dealer, but I think Napa might have it.

If you want some good exploded diagrams, I'm sure Scerb will chime in soon with his page on carbs. I just forget the link at the moment.

HTH