DTC 160 and 300

I have a 99 K2500 Suburban with 7.4 Vortec. Had some minor performance issues and recently some rough idle. This past weekend I thought I'd start adding some performance upgrades and decided on replacing the stock muffler with a Flowmaster 50 series SUV muffler. The truck ran stronger but still had some indications of a miss and stumbling while at stop sign. This morning the SES light illuminated and the dealer told me I had codes 160 and 300. Anyone have any simular issues after installing a high flow muffler? Any ideas regarding the stumbling or miss of the engine? Just for reference had the injectors, throttle body, tranny, etc flushed and serviced. New plug wires, plugs (Bosch +4), cap, rotor, hoses and belts. All done this past July.