You are looking for a 1/18 scale body, which I am quite sure does not exist in plastic. At present the best you can do is modify one of the ERTL 1/18 die cast Buicks.
Dylan Contact Steve Kohler. Star Models. stevekohler@cox.net. he is a great guy and can 3d resin print these 69 Skylarks. I did a couple 1/32 scale (for slot cars) and 1/25. I'm pretty sure he can print up to 1/18 scale. They are very nice, but don't expect them to be as slick as plastic model kits. They will need some sanding and detailing.
You can have almost any scale resin 3D 69 Skylark GS body made for you by Micron Modeling. I payed them to produce their own and had them make a few corrections and add more detail. You can get them on Etsy or eBay. They listed the kit with the incorrect hood but have since made the correction and will hopefully post newer pictures. George runs the place and is a good guy. Tell them Jake sent you.