Here's how would approach this project if it were me... And I'm an old Pontiac guy from way back so this project really interests me. I've had in the neighborhood of 15 GTOs in my time including a '69 Judge.
First I'd give it the Comet treatment to remove the dead and oxidized paint.
I'd follow that with either (or both) a #7 treatment and/or an oily protectant treatment. Mineral oil or silicone oil is also a good option for this step. The old paint will drink up the nourishing oils and prepare it for polishing. Repeat this step as many times as necessary, until the paint is no longer absorbing the oils or at least until the rate of absorption has significantly slowed.
Next I'd use a fine finishing polish. Something like Meg's 210. How much actual polishing you'll be able to do will be dependent on the paint's condition and you'll need to make that assessment as you proceed.
At this point the paint will be reconditioned as well as it can be given its age, condition, and type (acrylic lacquer from the factory).
For protection, you'll really need to experiment. Many typical waxes/sealants can be a problem on old, compromised paint. Anything that dries to a haze should be avoided as they will turn the finish "chalky" in a fairly quick time frame. Some will actually cloud up the finish immediately, others may take a few days, or weeks.
I've always had the best results using an oily protectant, in lieu of a conventional LSP, as the final finish for situations like yours. If you want to try a more traditional product, I'd try Meguiar's Ultimate Liquid Wax as it is very oily for a wax and it doesn't dry to a cloudy haze (relatively).
Good Luck! I'm jealous!
And forget the clay bar... Totally unnecessary for this project.