Re: Headrest question
From first hand experience with my 67' headrest seats I can tell you that the "receiver" hardware does not come welded in the seatback frames, but the correct hardware (which is spot welded in the seatback frames in those with the factory option) can be bolted (in place of spot welds) in / nest perfectly to the contour of the upper vertical bridge stamping. The bracketry still has to be obtained from original seat backs (typically only available from cars that were originally equipped with headrest and burned or a convertible that the seats were totally deteriorated from environment exposure), but in those cases the headrest themselves are trashed.
There are "reproduction headrest" being marketed, but it doesn't take very close inspection to tell that they're reproductions. The reproduction processes and materials used to produce them are "so nice" that anyone who knows a genuine 67' headrest will scoff at them. The only part of a reproduction headrest that could pass on inspection for the "real McCoy" is the chrome stem...
A couple of things to take note of is that in addition to the female receiver (with spring steel retention/positioning unit) there is a unique bracket spot welded on the very top of the seatback frame that the Chrome Escutcheon attaches to. NONE of this is common to 68'/69' (just as the 67' headrest are significantly different / taller than 68'/69' headrest.
I recently sold my 67' Camaro with factory headrest, but I still have in my collected spare parts one passenger seatback frame complete with hardware (no cover or foam) and two headrest [one needing a little more restoration / vinyl dyed, but surface covering still intact in decent shape] that I intend to sell. I also have pics of all the hardware removed from a seatback [not sharing the pics... but will be made available to whoever I end up selling my spares to]. If you know what you're doing / have some reasonably good fabrication skills it is more than feasible to replicate the seatback frame hardware with a combination of purchased reproduction parts (that will require some rework themselves). I've researched / investigated this to know exactly which parts to obtained and where (because I knew that a driver's side to complete my spare set would need fabricated given it would be highly unlikely that I would find a used one).
It all comes down to how much are you willing to spend and how much work/time you are willing to put into them... [as well as what is going to meet your threshold for originality - for me nothing but what came out of the GM assembly plant in 1967 would do, but if all you require is 67 vinyl covers on seatbacks with some headrest of some flavor in them then 69' parts will be the least expensive alternative, but won't add any value / desirability for your car to others, but if it pleases you than that is what counts. Recently there was a near stock 67' on eBay with gold interior / headrest and it was obvious the seats were 69's recovered with 67' covers.
If you want a true original factory look & feel there are no easy / cheap paths to it. 68'/69' headrest units can be differentiated from 67's 30 feet away/outside a car and reproductions from 10' away.