2001 Chevy Silverado P0300 and P0430: Stumbling Engine, Noise from Bellhousing, Fuel System Diagnosis

I own a 2001 Chevrolet Silverado with 110,000 miles. The vehicle has been making a bolt-in-spinning-drum noise originating from the bellhousing area. Several people suggested it could be due to a cracked flexplate or a failing torque converter. After removing and inspecting the transmission, I found the flexplate to be in good condition. As a result, I replaced the entire transmission with one that has half the mileage. The noise persists—possibly from a sticking starter—but after replacing the transmission, I also performed plug replacement, spark plug wires, and a PCV valve service. A few days later, when starting the vehicle, the check engine light came on. The engine ran smoothly with no hesitation, stumbling, or transmission slippage at that time. However, about 5 miles after refueling at a gas station, the engine began to stumble—sometimes severely enough that I feared it would stall. A friend scanned the system and reported codes P0300 and P0430 (Bank 2). I reset the spark plug gaps, replaced the wires again, but the check engine light turned off while the vehicle still exhibited hesitation and stumbling under load—especially when accelerating or climbing hills. I did not clear the fault codes. I tried two cans of SeaFoam: one in the fuel tank and one in the vacuum line. The symptoms have improved significantly but remain intermittent—sometimes the engine stumbles, sometimes it runs smoothly. The issue appears to worsen during acceleration or uphill driving. I consulted a local dealer who suggested that Midwest gas may be causing issues and pointed toward a potential fuel pressure regulator problem. While the check engine light appeared before refueling, no symptoms were present until after filling up. I plan to replace the fuel filter and perform a fuel pressure test, though it is currently snowing heavily. The engine revs fine at idle but stumbles under load. I would appreciate any additional insights or suggestions on diagnosing this issue—especially related to P0300 and P0430 in a 2001 Silverado. Please let me know if more details are needed.