Voltage problems, cutting out when hot

Is there anyway to keep testing the under hood connections bypassing the starter switch wiring at the column? Until I get a new starter switch installed?

Also I keep reading that aftermarket switches may not be plug-and-play, like they are one size fits all? But will need to be modified per vehicle year?

If there is a recommended source for getting a correctly pinned ignition switch harness for a 1974 please let me know.