It's fun to armchair quarterback something like this, but don't forget--this was 55 years ago. Even today, concrete is the better option. Steel would've rotted in those conditions but regardless, even steel ships of that era leaked from the day they were launched, which is why they had bilge pumps. Septic systems far newer than that have leakage problems; one of my neighbors has a fenced-off spot in her yard she calls "the shithole" because of a leaking septic system that's 25 years newer than that car.
Shoulda, coulda, woulda. Oh well. Overall, I think the car looks pretty good for being buried for 50 years.