Russ,
You are good with your memories. But they are older than mine -- we moved to Pasadena in 1955 during the summer I turned six.
Yes, the Pony Express Museum was on the south side of Huntington Drive opposite the Santa Anita Racetrack. I think the Pony Express Museum was located in the land now separating the east and west bound lanes of Huntington Drive. Between the old Pacific Electric and the SP Duarte branch right-of-ways. Some of this land between the lanes is now occupied by a hospital and also by some civic buildings, including the local history museum.
I should wander into the museum and ask them for specifics (we'll see how good they are with the local history). They once had a changing display with pictures of the big estate of one of "Lucky" Baldwin's daughters; it included a photo of the nearby, old Santa Anita AT&SF station and also one of a storage shed for her private car. I asked where the storage shed was located and the response was unknown -- all the photos came from the collection of the Huntington Library.
I remember the EAT-AT-EASTON'S sign -- as I recall that was at the corner of Colorado and Michilinda (next significant road east of Rosemead, a state highway). After Eaton's closed a whole new restaurant was built at the location -- today it is occupied by a Coco's Restaurant.
Brian