My friend bought a 1989 5.0 GT. It had no cats just a pipe to a pro chamber and 2 stub exhaust pipes. The engine is stock. It ran great when he brought it home. Then he had a new exhaust system put on, they "forced" him to put cats on. As he drove away from the shop he found the engine won't run at over 1/4 to 1/2 throttle. At rest it will rev free if you just roll on then seems to choke at about 1/2 throttle. If you are driving and the engine has to pull a load, it dogs out about 1/4 throttle. We are sure its related to the exhaust but not sure where to start. It was built with cats so why isn't running right? Its back to stock, new plugs, wires, etc. we disconnected the battery for a while to see if the ECU would reset but no change. No codes. Any ideas out there?