Air volume flow control

Sorry this is going to be a long post. My house is 1 and 3/4 stories. Built in the late 1940's with an unheated crawl space. The house was originally centrally heated with coal, then converted to fuel oil, and finally converted to have ductwork and a forced air natural gas furnace. The furnace is now a down draft high energy efficient unit. All the heating ducts run into the unheated crawl space and have been insulated by ourselves. There is only one zone for the whole house, so any control of air flow is done in each run off the main bonnet. There are only 8 6-inch diameter runs in the whole house. Every fall and again in the spring time I have to crawl into the unheated crawl space to adjust each heating lines damper to either reduce or increase the air flow to that area in the house. In the spring we restrict the air flow somewhat in all the runs for the main floor so that more air conditioned air is forced upstairs where its hotter in the summer time.