For someone is not accustomed to see much in the way of floor pan rust like this, how did it get this way? Outside in or inside out. I understand some rust under a leaky heater core but this car has rust all the way behind the front seat.
When I bought that (B7 Blue with white interior) 71 SFGT around 1995 it was being used as a daily driver and family transport vehicle by folks who lived in Northern New Jersey where they use road salt. I was able to store it indoor most of the time that I had it before I sold it in 2005. It needed quarters, a trunk floor, the fenders needed patching and there were significant rust "bubbles" under the vinyl top so it may have needed a new roof skin. The car now in the hands of a serious collector who has another 71 SFGT and a 70 6BBL GT so I expect it will get a proper restoration one of these days.
I think the this triple green 71 GT is worth $7k - $8k. Just my opinion. I know of another green one that is for sale but not advertised that needs a total restoration that the owner is looking for $10k for. Besides these 2, where else is there one for sale? Anyone who wants one and knows of one for sale is keeping it to themselves because they are so hard to find. I personally know of 3 of them that were junked and the survival rate for these cars seams to be very low.