I want to download harmonized files for 2010, 2000 and 1990. It’s easy to find census fiiles for those years, but nothing waves a flag and says “this file is consistent with …” Just need basic household and person data for tracts and block groups, like old STF1a. The harmoized files should be extremely useful, but it is hard to penetrate how to find them, as distinct from original format files.
It sounds like what you might be looking for are the IPUMS NHGIS time series tables. This tool links together comparable statistics from multiple US censuses in one downloadable bundle. Much of the information about how to access these tables and differentiate them from the standard tables can be found in the previous link.