I should do an update, I forgot about this thread.
Last spring I called the NJ DMV and requested a title search form which was mailed to me. I filled it out and sent a check to the DMV. I got a letter back a month later telling me that they had purged thier records and that there was no indication of the car being titled or registered going back to May 1988.
Now that could mean the car was junked by then, but it also could be unregistered or titled sitting in a garage, yard, field or it could also mean that the car was taken to another state by then.
It would get costly doing searches in all states but that becomes another problem too. Some states now no longer issue titles for older cars so I wouldn't be able to do a title search on a 1973 in those states.
I know that my chances of finding the car are almost nonexistent but it doesn't hurt to try and look every now and again. That's why I have the VIN in my sig on here, the Vintage Mustang Forum too, watch Ebay for 73s at times and have even been contacted by a few people every so often about a car they saw that I will check out.
The Chevrolet dealer (Artsma) that my father traded it in to has been out of business for years so that's a dead end too.
The Grande was well maintained and garaged it's whole life and in very nice shape still when he traded it in and unless it was in a wreck had plenty of life still left in it. His job moved further away and he wanted a better fuel economy car when he found a 4 cyl Citation that replaced the Grande.
I got a deluxe Marti report on it too. That's when I found out his car was rather unique. It was Dark Metallic Green with Ginger interior, 1 of 139 Grandes in 1973 with that combo. The car's color scheme was identical to this convertible
http://www.svs.com/zim/mustang/images/73-4Q.jpg