I would just worry about speed bumps with a 4-1 header, they seem to sit pretty low. I would just install everything at once, that way you don't have to take them apart twice. I would PB-blast the nuts that fasten the header to the head now and do so at least once a week until you install the header. Trust me, you don't want to break a stud. Although this isn't a very common situation, it still happens. Also, buy new bolts and springs for the header-cat joint.