Talk to Mogoat on here about body's shop "jail" too......lots of abandoned projects out there because an over enthusiastic owner got in over their heads or the body shop went belly up.
Not trying to scare you off but for a run of the mill GTO if it's too rusty it is better to find a solid car.
Now if it was a RA IV.......
I have read all the horror stories and looked at a lot of overpriced pieced together Bondo queens for over a year looking for a '67 and found a Ca. car that was rust free that the second owner paid 25+ just for paint and interior.
Cosmetically it was a 10, mechanically about a 4 but for the most part it was original and unmolested.
I probably paid too much but I did get the seller to come down 16K.
I did most of the work myself, under the dash was a mess, I rebuilt the steering column, restored the steering wheel and the heater core was leaking so I replaced that and had to source another A/C heater box that someone had cut up with tin snips to remove the core from the inside.
I also had to re glass the plenum which had cracked and became brittle with age.
Both the original and the one I sourced were in bad shape.
Going through the dash was time consuming and would be expensive if you could find someone willing to do it.
The suspension needed everything all new suspension parts and I added boxed trailing arms, Bilstien shocks and upgraded the front drums to disc.
Then there are the incidentals, harnesses, vacuum cans, gaskets, mounts, PHS report, shop manual etc.
I also replaced the J.C Whitney gauges with Greenlines and a retro sound system and speakers since I had the dash apart.
I went through the tranny at my buddy's shop, new pump seals, pump bushing, clutches etc.
These parts were cheap but again not so cheap if you have to have a tranny shop do the work.
All that including going through the rear end and rebuilding the driveshaft is done.
Exhaust and new tank is done, Pypes system and I traded boatwork on an outdrive for the welding.
Then there is the engine it has to be rebuilt if you want it to run on pump gas.
5 new tires and another Rallye II rim for the spare since the 14" rim would only work if I got a flat on the rear.
It is all done now just waiting on the engine from the builder.
I have spent the 16K I talked the guy down so I was pretty close to what I told him it would cost but it would have been a lot more if I had paid to do the work instead of doing most of it myself.
I am just waiting on the engine from the builder so the cost of the car restored bumper to bumper is right at 61K including the Ca. tax and DMV fees.
It will look like it did when it rolled off the line in Fremont in '67 but it cost a bit more than the original price of $4,105.87.
If you can't do the work I would pass on the car and try to find one that is in better shape, finding a shop to do the body work is going to be nearly impossible to find at any price.
I went through this with another vehicle (body shop jail mentioned above) and had to get it back and do the work myself and buy the equipment to do it.
I got ripped off for 4K and 6 months of time (estimate was 4-6 weeks) while it sat outside and I got it back with some of the parts missing I had to source and pay for.
That is why I bought one rust free with all the bodywork, paint and interior finished, I probably saved money in the long run and I got to inspect the work up front.
4K sounds like a good price but if you can't do the lion's share of the work or have a large bankroll I would pass and keep looking, you don't want to have 25K sunk into a project that is half done and nobody willing to finish for you.