In NHGIS you can use the filters from the main data finder to enter these criteria and search through available tables.
For example, I entered the following:
Geographic levels : County
Years : 2010
Topics : Race (topic filter), Household & Family Income (topic filter)
This will create a list of tables meeting the above criteria and should help you find the median household income at the county level by race if the table exists. This is the first few results from the specifications I outlined above:
Note that you may also want to try your search with race as a table breakdown filter instead of a topic filter; this will capture tables that don’t include race as a topic, but instead offer iterations of unique topical tables for each race group.
I did not see tables meeting your criteria in 1970-1990 in my quick searches upon reading your post. After not turning these up in NHGIS, I also searched directly from the Census Bureau, but didn’t see tables that included race at the county level. If the underlying tables do not exist, there definitely won’t be a time series table through NHGIS.
If you are interested in creating your own time series tables generally, I would direct you to this NHGIS summary documentation of both the nominally integrated tables and geographically standardized tables as well as detailed methodology used to create the latter.
While summary data available through IPUMS NHGIS will allow you to capture lower levels of geography, the microdata from IPUMS USA will allow you to create custom tables if the information you are interested in is not included in any of the summary tables that Census created.
I hope this helps.