Well what you think it will run and what it will run right now are two different things. Those numbers say its a 13 second car. Get a walbro 255, they are cheap and better than what ever stock set up you have. But really you need to log some data and see whats going on before you start throwing parts at it. Also, just curious do you have a 240z or a 280z gas tank? I had starvation issues with a non efi tank because it lacked the baffles.
My set up is kind of wonky, I started a turbo build and then went to nitrous. The motor is an iron block 5.3 with 243 heads with aftermarket springs and push rods. A ls6 intake and the cam is a small custom grind reverse split. On motor it puts down 300whp through the TH400, which is pretty good considering its a low compression set up.