In over 50 years I have never seen a blocked line inside a tank.
I would suggest putting a vacuum gauge on the pump to make sure it's pulling and then move the gauge to the hose at the sending unit and repeat the test. If you have the same vacuum reading then you have eliminated everything to the tank
You didn't say what motor, type of pump etc. you're dealing with so I'm assuming it's a mechanical pump.
A loose or rotted hose or a pinhole in the line (rusty) will wreak havoc. If it happens to be a big block it could also be the fuel pump pushrod.
getting the car up with the rear end hanging will give you much better access to the sending unit if you do wind up going there.

keep us posted