Basicly there were no real chassis spec's at that time, and for whatever reason it was changed/built like that, no right or wrong about it, its just the way it was.
If you find a chassis with the "hour glass" seat support, not that big of a deal to change to the single uprite design pictured.
"Needle in a haystack" quest trying to locate your dads car, its been over 50years and 30 years since the nostalgia/vintage craze started, many have been scraped within that 20 year span, hopefully not your dads.
Personally (since I do this) these cars were very simple and easy to replicate. It takes either luck or years and money to find a car if its still out there, Sitting by the phone waiting for a call will never happen.
Get all the pics you can find and hand it over to a capable restorer and have it duplicated (at least the rolling chassis) and start enjoying it sooner.