1976 Toyota Landcruiser FJ55 350 Carburetor is running out of fuel when under a load

1976 Toyota Landcruiser FJ55 w/chevy350 Carburetor is running out of fuel when under a load. I have stopped the vehicle as soon as the motor leans and quits. Checked the carburetor fuel bowl (350 Holley) and no fuel. The truck has 2 tanks and a mechanical type switch. This happens on both tanks. I have tried 2 pumps and 2 separate lines from the switch to the pump. The pump is firewall mounted. Turning the ignition on after it stalls gradually fills the bowl again for a restart. The pump is giving about 60 liters per hour from a basic flow test I did. The voltage seems ok at 13.5 volts and steady. I have checked the needle and seat, cleaned out carburetor etc. It idles fine, accelerates fine, but just runs out of fuel when under a load for a while. I'm suspect of the needle and seat, as I can't see what else it could be. But the needle seems quite free and the float level is correct. There is a lot of fuel pressure right at the carburetor inlet after it cuts too. I'm baffled. HELP! Mark