While prototype fidelity may not be overly important to you, it helps to know the history of FP45s on the ATSF so you can make informed decisions on the cars you decide to pull behind it. They arrived late on the ATSF in passenger operations in 1967 and were assigned to the combined El Capitan / Super Chief initially but did pull other passenger trains throughout the system. Either double level El Cap cars or single level streamlined cars would be appropriate pulled separately or combined. 1967 was the year that most postal contracts got pulled from the railroads, so the RPO would not be prototypical, but pretty much any of the other streamlined cars would work for you. A minor point also, but most if not all ATSF observations got rebuilt to be blunt ended for use mid-train so a round end obs wouldn't be correct for that era of ATSF. However, it would still look good.
I have a full set of GGD El Cap cars, five of the Lionel 21" streamlined cars, and a few K-Line ones. Combined they make a really credible El Cap / Super Chief. If you can find the Lionel cars at a good price, I really like them. They are plastic clones of the K-Line cars and often can be found a little cheaper and they have LED lighting.
Also keep in mind that Amtrak leased the FP45s for the first few years prior to taking delivery of the SDP40Fs in 1973 so it wouldn't be uncommon to see them pulling a mixture of Amtrak and ATSF cars on the Chief.
Here is a video of some of my FP45s pulling that train with the K-Line cars. I broke all the rules and added some heavyweight cars to the head end and ran mine with the 63' RPO. Also note that one of the FP45s is the later paint scheme which isn't correct either unless it's pulling a business special. So much for prototypical accuracy!
Here are some photos of Lionel's 21" cars.
