Never assume anything...

[HIGHWAY BY THE SEA]

Today I got my 67 GMC 3500 back from the muffler shop. It was dropped off there immediately after I got it back from a month-long stay at the brake shop. I was jones-ing to work on this truck after so long a separation, so I did work on it all day.
One of the first things that I did was replace the fuel sending unit with a brand new one that I bought several years ago, but never used.
When my son got home we got into the truck and drove towards the gas station, since I had noticed that the gas tank was a little low when replacing the sending unit.
We did not even make it five blocks and the truck just DIED. It would not start, so I popped the hood and checked the fuel filter. Half-empty. I figured that maybe the gas level was lower than I had initially thought, so I walked back home, got my 5-gallon jerry can, and drove in my car to the gas station that we were enroute to earlier.
After filling it up, then transferring most of the gas into the 67, I tried to start it again. Again, no gas was in the fuel filter. I tried priming the carb, thinking that maybe there was a big air bubble in the gas lines, but again, no gas. I then realized that there was a blockage in the gas line somewhere, so I drove my car back home and got my tool box. Upon returning, I unhooked the fuel line from the fuel pump and gas tank, then blew air into it. It went through with no issues, so I reattached it to the sending unit, then blew air into it from the disconnected fuel pump line. I could not get any air to go through it this time.
Even though that I knew that the sending unit was in new condition, I checked it anyway. With the fuel line detached from it I tried to blow air into the sending unit tube. It was totally BLOCKED, and I have no idea how this happened.
So I drove back home yet again, grabbed the old sending unit, and upon returning to the 67, installed it back into the gas tank. This time the truck started.
Today I learned never to assume that a new part is good, but I still have absolutely NO idea why it was clogged. I tried to unclog it with a piece of stiff wire, but only succeeded in getting a little air to hiss through it.
I was wondering if anyone else on here had even encountered a problem like this?