Deans Garage has posted a treasure trove of GM design studies from the 1960s through the 2000s, grouped by designer:
David McIntosh: http://www.deansgarage.com/2015/davi...h-gm-designer/
*Early variants on Cadillac Art-and-Science theme. Including a very small (front-drive?) model and a very large one (DeVille/Fleetwood replacement?).
*All sorts of wacky early efforts at what became the Lambda models.
*A Lumina replacement that might have given the Chrysler LH a run for its money.
*A 4WD Lumina APV.
*And much, much more....
Clark Lincoln: http://www.deansgarage.com/2015/clar...n-gm-designer/
Includes mostly 70s and early 80s small and midsized studies. They all look about a decade ahead of their time.
Allan Flowers: http://www.deansgarage.com/2015/alla...s-gm-designer/
Mostly front and rear details from the '60s and '70s but spiked with some interesting small car proposals.
Gray Counts: http://www.deansgarage.com/2015/gm-d...r-gray-counts/
A lot of advanced concept work, much leading up to the early '90s Achieva/Skylark programs.