95 Impala stalling

I have a 95 impala ss with 150,000 miles mods are team ss air cleaner home plate delete first base delete. I currently have an exhaust leak where the manifold meets cats. After driving 40 or so miles the check engine light comes on and the car stalls and will not immediately start. Upon trying to start there a strong gas odor and the car will eventually start a repair shop said codes were no help. Im not sure exactly what codes came out. Should I buy a code reader myself or should I be replacing the most likely candidates maf, o2 sensors, egr, pcv, or may exhaust leak be setting knock sensors retarding my timing.I could really use some sound advice. Any help would be greately appreciated.