P0420 Fault on 2001 Ford Lightning After Incorrect Muffler Installation

I own a 2001 Ford Lightning with 54,000 miles and am currently experiencing a P0420 diagnostic trouble code. Initially, the vehicle showed a P0172 code, which I have since resolved. The P0420 appeared after replacing the exhaust system with a MagnaFlow muffler. This replacement was problematic because the original setup required a dual-in, dual-out design with side exits—specifically, two 5-inch tubes (totaling 2.5 inches in diameter) to maintain proper exhaust flow. The new MagnaFlow unit has only one exit, which is reduced from 3 inches to approximately 1.5 inches due to a sharp bend and stretching during installation. This change significantly restricts exhaust flow. As a result, the vehicle now struggles to maintain engine performance above 2700 RPM—experiencing a noticeable loss of power and a 'bogging' sensation. Additionally, I recently detected a very minor exhaust leak on the passenger-side manifold, only audible when the engine is cold. This issue did not exist prior to the muffler installation. When taken to Ford Service, they observed a powder-like substance emerging from the exhaust—possibly indicating internal clogging or material degradation due to improper fitment. The vehicle is supercharged, and any restriction in the exhaust system will have a direct impact on performance and emissions. I believe the root cause lies in the incorrect muffler installation and inadequate exhaust flow design.