Didn't have a clue, other than the Corvette part. I guess it must be that the Corvettes get most of the attention, it never occurred to me that other car were being built in St Louis as well.
As far as Muncie 4 speeds in Impalas, I bought one early in the 70's from a guy and it came out of an Impala with a 396, as I recall. It was either a 66 or 67 model.