What Was the GDP Then? (Measuring Worth)
This site is from economic historians Louis Johnston and Samuel H. Williamson. They use a variety of sources for this project that estimates U.S. GDP from 1789 to the present. As one might guess, reliability is an issue, and they state, "A central point to keep in mind is that the quality of the data deteriorates the farther back in time one goes." They provide very extensive documentation.