Dave, about 10 years or less? I saw a gs like the one you are describing at a swap meet in Elkhorn, WI for sale!
When I first saw it, I thought whats up with this car?!( Halo roof , gs badges?) The guy who had it for sale was from either around Milwaukee or up by Eau Claire
WI!? I believe he stated that the car was from out west or Texas? After closer inspection the car was a black ( firewall tag ,back of doors etc) z code ( ca built I think) stick (shifter plate and pivot welded on the frame ( boxed) I found no holes or pins in the doors, fenders, or quarters if it was a re bodied car! which it was not! It had no drive train, no interior , needed floor and quarter work, still had the stick pedals! I did not get to look under the filler panel to see if the gs trim hole s where filled in and how the halo trim was put on?! Hope that this car is still around and you can find it!

Do you have the vin ? , church records to find the pastor? Jon