FWIW, If you're still working on that thing. The long timey ago first field repair to these old flathead fuel pumps was to put a tight fitting piece of leather up in the arm of the pump. You know, where the push rod fits. As the pumps wore, the pushrod wore, the cam wore, you sometimes lost enough movement to the pump. I've heard of pumps being rebuilt with diaphragm, valves and still not work 'till the leather shim was added. The leather will provide just enough additional "carry" to the fuel pump arm to make a difference in some cases. I've seen it work.
But, if the pump is bad, this ain't gonna help.
Sorry I am late to this party and if someone has already suggested this I am in the apologetic mood.

 

I can't think of a check ball that's in there, or why there would be one. The pump itself has check valves.

I'd seriously recommend using coated steel brake lines from NAPA to replace as much of the hose as possible. That long of a run in rubber is dangerous and the pump suction may very well be collapsing the hose. I used two pieces of brake tubing, with a connector right where the line (is supposed to) pass thru the corner of the front crossmember. Basically one piece goes from up near the pump, down the firewall, and turns back along the frame. The other goes from there to the tank shutoff valve.

 

solved the check ball question. Its not supposed to move.

Looking at NAPA right now Ross.....what size line would be good?

 

According to Ross's diagram that pump used two disk type check valves rather than ball type used in some later pumps.
I'd use the largest line that the fitting fits the tank and pump, likely 1/4". Take the pump with you to the parts store to check. There will need to be a short section of rubber line (~4"?) typically somewhere near the pump to allow the line to flex when the engine moves around. I'm sure Ross or Ilya can tell you how long and where it went originally. Be sure to use hose clamps to seal it to the metal line. The rest of the line should be clamped to the frame with tubing hanger clamps so it can't vibrate. Be careful it doesn't rub should it pass thru a crossmember. If you don't have one, you may want to invest in a tubing bender to get neat smooth bends without collapsing or kinking the tube. They aren't (or shouldn't be) expensive and can be used on various sized tubing. Nothing looks better than metal lines with consistent radius bends and perfectly straight sections in between. Use a piece of straight stiff wire to mock up the tubing shape, then bend the tube to match. 1/16" brazing rod is commonly used.