I kind of hate to answer questions like this, even though I know the correct answer! The answer is BLACK. Both Buick and Olds were supposed to spray underneath the cars BLACK. It's in the assembly manuals, I worked in gas stations in the late 60's and early 70's, and I can assure you that all these little paint slashes and marks people do now, you couldn't see then. The back? The gas tanks were black, shocks + tail pipes had black overspray, A LOT!!! AMES has a 4K 72 GS in his collection, I slid under it and what I'm telling you is the way it was. This car had 4K miles, and probably would get absolutely hammered in the GSCA concours class. I know...you see them with cast iron color parts, maybe black adjuster sleeves or whatever. You see them with pretty, pretty copper cotter pins...it was all BLACK.
We accumulated the firewall forward finishes here: http://www.v8buick.com/showthread.php?55559-Firewall-Forward-Finishes-and-Markings-Part-Deux
