There is a nice 1969 third row seat for sale on Chevelle Wagon Facebook page, located in Wisconsin.
You can put a 70 wagon or El Camino frontend on a 68 or 69 wagon, too. But I guess, you wouldn't get a dual action tailgate, that way.
66 El Camino 396/325HP 4-Spd
My Build Thread:
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62 Corvette 327/360HP (original RF engine and Fuel Injection) (future restoration project)
66 Corvair My son's project. Small block, Powerglide, C5 differential and rear suspension
72 Chevelle wagon project