First off, I would not recommend installing one of the indoor-only WCT 36" or 42" models outdoors as they are constructed of galvanized sheet metal and would rust rather quickly if left exposed to the elements. Instead, I'd recommend a stainless steel outdoor system like the Superior "WRE" series in a 36" or 42". Models like this are shown here.
As far as the 20 lb. tank you plan to use, this is common for smaller outdoor fire pits that will be used with little frequency. If you plan to use this log set with any frequency, I'd recommend using a much larger tank to avoid any headaches resulting from frequent outages. While this is more problematic in colder weather, LP tanks are filled to roughly 80% and will only absorb heat through what is known as the "wetted surface area" or the area inside the tank bathed in LP. As LP gas is depleted, the wetted surface area also decreases, along with the pressure the tank is able to produce. So a 60,000 gas log set installed with a 20 lb. tank will use just over 2.75 lbs. of propane per hour. After only 2 hours of operation with an ideal temperature of 60 degrees, the tank would "freeze up" or not be able to provide enough pressure to keep up with the input rating of the log set.