1984 Ford Mustang not running right and stalls under 2500 RPM

1984 Ford Mustang mileage: 1500. I have recently installed a modified 1967, 289 into my 84' GT. I had the block bored 40 over with Keith Black hypereutectic pistons. My cam is a Comp Magnum 270h, which puts out about.501 lifts w/ 270 duration. I have the 289 hi-po heads port matched and polished w/ a Weiand stealth intake. A Carter 625 runs the fuel. When I start the motor, the engine will jump to about 2500 rpm, and then if I try to kick it down, the motor stalls. It will also stall on its own if I let run. Initially, I assumed it to be either a carburetor problem, or a timing problem, but I have reached the end of my rope and patience. Everything has been checked two times over, including manifold leaks, fuel pump, fuel filter, cam lobe wear, compression, and timing, and still no results! The last time I had the car running, however, I did notice that four of my cylinders weren't fully firing until the rpms were kicked up (I gave it the old spit on the header test), and I was continually checking my plugs and the spark to each cylinder, and all were good. Any help would be greatly appreciated!