Classic/Antique Car Repair: headers, carbon gases, ems code

Question
hi;
 i have a general question regarding installation of exhaust headers on a '76 Nova with a 305 motor.Why are headers a off highway item and not designed for on road emissions vehicles?Arent headers longer versions of oem cast iron exhaust manifolds?
                             thanks

Answer
I did not know that you could not have headers on that year car ? or that they do not have headers that will meet the ems. code for insp. I had headers on all of my street cars up till 1980 with out any trouble and that is ON the street but with a set of good header mufflers to keep it quiet but if you hit it , the noise was  loud like they were open. I still see guys that have them on cars, I would have the stock ex. system on it get it insp. than take the ex. off and put the headers on , it is not to hard if you have all of the equp.I would look into a set that might be street legal , try -JC Whitney & co. if thay make them they will sell them.The headers are tuned for the engine and bent for the best  and fastest way for the sound , and carbon gases to get out and with the proper back psi. as they do give you a large boost in the horse power .so I hope that you can find a set that will work for you as you will see a big change with them on it. Good Luck
Roger JM. W.