If so, what variables should I use?
Counties are actually unavailable in public-use microdata from 1950 onwards. The counties that are identifiable in IPUMS USA (via COUNTY or COUNTYFIPS) are only able to be recovered (by IPUMS) due to the existence of other low-level geographic identifiers.
Although I am not certain of the specifics of your work, one alternative may be the data available through IPUMS NHGIS. If you are looking for data aggregated at the county level, then this IPUMS project should be useful to you. Since IPUMS NHGIS provides aggregated data, rather than microdata, it is not subject to the confidentiality restrictions that limit county identification in IPUMS USA. If you are looking for full coverage of counties within US Census microdata, you’ll need to reach out to a Research Data Center for use of restricted access data.