Another great source to gather information, both on the dealership as well as the people who may have worked there would be to contact the Iowa City library (information desk) and newspapers.
Provide them with as much information as you can on what you are looking for, names, ages, addresses, etc. from your existing information and ask them to check for newspaper articles, ads, etc. from the time period. Also ask that they check the phone books, city directory and the Chamber of Commerce for any contact records/information.
Depending on the person or city, they may or may not "be able" to help you out, but it is worth a try.
I'm doing my own research on my Kaiser based car and have tracked down all but one owner/family and spoken with the a number of designers and/or their families in the past year.
Even with the great technology available to us, I've been unable to track down any of Howard A. Darrin's' family. I've found 2 distant (4th and 7th) cousins, but they had no contact information and only the knowledge that they were related to a him.