Regarding can't remember where parts go 58 Impala
well I finally remembered (figured out) where these pieces go. The part #'s on them are RX0 AND LX7. They are attached to the back of each of the upper interior side panels with staples. They form a little rounded area which forms the back of those upper side panels and tuck in behind the upper 1/2 of the back seat. There is a convenient curve on the back side of the seat which accommodates this now attached "filler panel". Once attached with staples they just hang from the back of the upper interior side panel and look like the fit nicely in their spot.
Thanks to all who burned brain cells thinking about this...Rudy