Is My Cuda Killing Me?

So I've had the engine and exhaust (as well as electrical, brakes, steering, upholstery, etc...) all rebuilt. However, if I take it for any length of a drive I get bad stomach pains, light headed and stink of fumes. After speaking to 3-4 mechanics they believe the exhaust work was sound but that - SOMEHOW - the tail fumes are coming back into her. Thoughts gang?