Just a thought - if you're going camping, fishing, etc it might be a good idea to have 12v for the trailer/boat you're towing around. Also, didn't they have dash-fans in those days? Recently there was a very nice 40 Chevy Sedan Delivery that had a dash-mounted fan. I don't think that's an unreasonable era-correct modification. It was once explained to me that switching to 12v was "smart" and a safety thing (not sure what they were referring to). When I was driving my truck back from California I broke down. AAA asked me if it was 6 or 12v for a boot. He said they would need a different vehicle for the 6v. Who would really notice if it was 6 or 12 anyways?
