Leo,
You are 100% correct that 1958 wasn't a leap year. Bad assumption on my part without checking the facts.
My apologies!
In zooming in on Jsalchenberg's picture of the casting code date marks, it's possible (probable?) that the rusty uneven cast surface built up in the area has made the first character appear more like a "B".
Take the rust onto account and that first character is more likely a "D", which would indicate "April" of 1958. April did have a 29th day (were February didn't) making it more likely the engine the engine was built in April 1958 instead of February.
Jsalchenberg,
Scrub off the surface of the machined pad just behind the distributor mounting hole and add a picture of the serial# stampings that should be on that surface to help better define what your engine is.
According to this
1958 Chevrolet Truck Data document, the engine serial number can be decoded to identify the engine and truck it was installed in originally (see screenshot of the extracted code chart below).
Dan
