OPGI was not particularly helpful. They could just tell me that the kit contained front, rear, and middle braces. I'd have thought the middle brace was the one I needed:
(random picture from the interwebs, don't know source)
I think the unknown "middle" brace is for a Chevelle (see this Chevelles.com thread with a pic).
Regardless - the set got returned. It does not appear that the middle
brace I need is being reproduced, nor the equivalent for the longer-roofed
vehicles.
Go back to the previous day (6 Nov 2021)