I wonder if that bus came with a gas engine when it was new?
Many came with 500+ cubic inch engines back in the 50's. It was not until around 1960 when they started using diesel engines on a regular basis, starting out with the 6V71 two stroke Detroit diesel. That engine was also used in train service, many times as a generator on the diner car. . .
Looks great! I bet it has a Detroit 71 series or 92 series engine in it now. . .
Fred.