You have a mechanical fuel pump. There is no sender unit. It has just a line from the front to the tank at the rear that should free flow the fuel. The pump has a arm the is moved up and down by a cam inside the engine that moves a diaphragm inside the pump to suck the fuel from the tank and pass it on up to the carb. I would disconnect the line from the fuel pump that goes back to the tank and connect on to it a suction bulb to try and draw the fuel through the line. It should flow pretty easily. If it doesn't, then there may be debris in the tank stopping the flow. Blowing back as you've done may momentarily move the debris so you hear bubbles, but it can quickly move back and stop the flow necessary for the pump. At that age, it may be prudent to drop the tank and clean it out.
If you are getting flow to the pump, reconnect the line going in and disconnect the line at the carb. Try cranking to see that the pump is working when you crank. You may have the wrong pump or installed it incorrectly for the arm to connect correctly with the cam to get the up/down action.
If you are getting flow out the line that goes to the carb, then you have problems with the floats or needles.