2002 Chevrolet 25-00HD 6.0L 4x4: P0102, P0106, and P0300 Codes - Diagnosis & Ground Wire Issues

I recently purchased a 2002 Chevrolet 2500HD 4x4 with a 6.0L engine. I've never driven it before, as the previous owner had removed the bed to replace the fuel pump but never completed the installation. The truck had been sitting for approximately two years and was reported to shut off unexpectedly when driven—often requiring the driver to bang on the gas tank to restart it, with some instances needing a cooldown period. I recently performed maintenance including replacing the fuel pump, fuel filter, and spark plugs and wires. I set the plug gap to 0.60 inches, which matches the manufacturer's specification for the original plugs. However, when cranking the engine, I notice severe misfiring and a strong lean gasoline odor. The onboard diagnostics show three persistent codes: P0102 (Manifold Absolute Pressure - MAF Sensor Circuit Range/Performance), P0106 (MAF Sensor Circuit Intermittent or Stuck), and P0300 (Random/Multiple Cylinder Misfire). Additionally, the ABS light remains constantly illuminated. I understand that P0601 indicates a memory check error in the Internal Control Module (ICM)—essentially the truck's main computer. I've read that poor grounding could cause erratic readings and potentially trigger these codes. Could a faulty ground connection be responsible for all three issues? Is there any way to diagnose or fix this without professional equipment? I have no prior experience with this vehicle, and the previous owner has been described as mentally unstable—this adds complexity to troubleshooting. Any insights or suggestions would be greatly appreciated.