Trying to think of ways to track the car down. Not much info to go on.
What color was the car?
How long did he own it?
Any old family photos with the car in it?
Who did he sell it to?
Where did he find the original listing for the car?
Where did he list it for sale?
I'm sure you have searched, but any chance of finding any type of paperwork related to the car??
Any specific special features about the car that anyone remembers??
You can try DMV but A. they might not still have records going back to the 70's or B. they don't give out such info.
All you would have to go in is asking DMV to give you info on a Chevrolet Nova that Arthur Richard Swanson owned in the early '70's. You cannot even try to get your hands on a NCIB report without the VIN. Also those are near impossible to get a hold of.
Keep digging! I am sure you will find out something which helps you find the car.
You could do like the Papa John's guy did and post a $200k reward!!