The first thing you have to accept there is that all of us who have responded have built cars, some of us a LOT of cars. Some of us, for a living, every-single-day for years.....building cars.
I know I have been doing this stuff for a living and as a hobby for 45 years, full time as a job for 41. Not always building cars, just regular old collision work too, but you get the idea.
Back in the seventies in a Rod Action magazine there was an article on building a car and what to expect. It gave one of the most profound explanations on how to figure out the cost I have ever seen. Now after all the years I have been doing it I REALLY have a lot of respect for this hot rod cost calculation I they printed in their magazine (remember those?)
"Sit down and figure out what it is going to cost you in labor, and parts and supplies. Then double that, then triple that! You will still be wrong but not nearly as wrong as if you went with your original estimate."
What we know looking at those photos is we KNOW we see lots of damage, but the most important this is by seeing what we see we KNOW there is a bunch more we aren't seeing!
If this were a 2008 Chevy the rust can be local, rusted right through and inches away solid. Cars have been fully coated in Zinc now for a few decades so we know that rust can be localized.
Back when that Firebird was made there was nothing of the sort, and when you see rust like that you KNOW there is a BUNCH more.
The shop that gave you those prices aren't making much if that is all they charge you. And that my friend is one of the biggest scams is giving you a low number then getting it in there and telling you there is more needed.
I used to do those cars when they were only a few years old and here in sunny California and the window channels were rusted through all the way around and up into the roof. I will bet you, right now, in front of everyone I will bet you a 24 pack of beer that there is rust holes ALL THE WAY AROUND the windows front and back. That is just a tiny little bit of what you are going to find.
If you want to do it that bad, go for it. But as I said, get another car and take a bunch of parts off that one and put it in the other car and paint it the same color and all and that will be your Dad's car.
If not, if you want to do it, right on, you have a LOT of work, MUCH MUCH more than you understand. But that's cool, if that is what you are ready for and willing to do, right on.
Just don't think we are exaggerating, don't think we are being all evil and unrealistic and not understanding your emotions and love and human feelings, we know all that too. We have all done these things, I am keeping my Gran Sport I really don't give a crap about anymore because of my son who drove it to high school 20 years ago. I am building my truck because of my history with it, I get it. I have many many pieces of art I have made out of family memories, I get it.
But there is a line to draw, that is a MONSTROUS project filling your garage and house with rusty dust, if you are ready for that, go for it.