I would go to the local autoparts store and specify new socket head cap crews in the correct length and thread pitch. They are less likley to round off and they will be very strong. Unless you know how to read the markings, you are taking a chance with anything else. Stainless steel would be best. In all honest, you might want to look into converting to a stud and nut combination as that will allow hanging the header more eaily.
Look at a farm supply store. Inn my experience they have the largest selection of bolts. Look for hardened (probably class 10.9) bolts in the correct size, length and pitch. Also look for a sturdy head.