I think that none of my 1954/55 Chevy/GMC cabs had captive nuts for the side mirrors. Yes. you have to be somewhat of a contortionist to hold a nut on the screw/bolt on the driver's side. You can take out the glove box to get at the threads on the passenger side (for the optional mirror on that side).

Here is a picture that Denny Graham posted a while ago.