Thanks, guysDealerships did offer an accessory kit to retrofit an overflow tank to '66-69 Buicks. It was GM part no 981766 which included the tank, wire cage for the tank, hose w/clamps, and radiator cap. These show up here and there on eBay and on sites selling NOS parts from closed dealerships etc. It also shows up in Olds parts catalogs of the early '70s. It was discontinued around 12/73.
The tank by itself was GM no 1239023; the radiator cap 6410570 or 6410665.
Factory installed tanks started showing up in 1970 for Buick.
Here is a thread showing installation of the tank on a '68 model:
Recently I purchased a NOS 1968 Buick Overflow Tank Kit on Ebay. (part number 981766). This kit appears to be specific to the 1968 model year according to the online Buick parts references I use. NOTE: The directions for this kit dont provide any recommended routing suggestions for the overflow t...forums.aaca.org