Highly recommend the Atlas 55T fish belly. It's available in the older round herald scheme, which should make up the majority of your fleet. The WM's fleet was limited to these smaller cars due to the limitations of the rotary dumper at Port Covington. As that was upgraded in the early 60's, many of the 55T cars got rebuilt into slightly longer 66T capacity. The 70t cars that Craig offers were introduced in 1959 or so. The speedletter paint scheme was introduced in mid 1954, so if you run a few of them you're good, just don't weather them much. There's at least one photo I'm aware of that shows a WM H-9 consolidation in the same frame with a freshly painted speedlettered car, so you have a little latitude to mix and match. (Speed started appearing in the spring of 54, steam was gone by the end of October).
MDC/Roundhouse also offers a 70T car that's a reasonable stand in, but again, it's a newer car than might fit your era.
The good news is that the car numbers that Atlas has used represent the cars as originally built. These lasted for decades, but many were rebuilt into newer classes with different numbers, although the outward appearance barely changed. (Most of the upgrades were mechanical)
You can read more about how I tinker with them here:
http://www.wmrywesternlines.net/rs_atlashoppers.phpYou can use the Micro Trains 2-bay hopper that's been done as a custom paint job, if you can find them. They're not accurate, but are a reasonable stand in for older classes of WM hoppers.
The black MT car that shows up now and again is an original Kadee car, but it's grossly inaccurate including the round herald on a black car. Prior to the introduction of the round herald, 1930s vintage hoppers were black, but apart from a few murky black and white photos that show a "dark" car with a round herald, there's no evidence that the logo appeared on anything but oxide red.
Hope this helps.
Lee
