Depending on what type of headers they are and what there going in. Some headers require a bit of gentle bashing to make them sit right .Basically its a case of taken the old manifold off ,scraping off the old gasket , bolting the headers on and simply weld the flange on.It shouldnt take more than 2 hours ,so going by your figures $50 bucks an hour sound about the usuall, may be slightly on the high side. I changed from Hurricane to pacemaker in about an hour,so someone in the trade with experence and airtools should be quicker. Its only 12 bolts and a weld.
Dave