I'm jumping into this tread late, but here's something additional to consider ...

In the early days, there was downtown Las Vegas (incorporated city) and the rest of Clark County (unincorporated).
Beyond the city limits, going south, the typical street name was just Hwy 91 or the LA Hwy.

When Paradise was created in January 1952, this unincorporated community may have had influence over name change to Las Vegas Blvd. South ... and by extension, a name change for 5th Street.
This is my speculation only. If you want to read background on Paradise, go to Wikipedia page: http://en.wikipedia.org/wiki/Paradise,_Nevada

Though transparent to most visitors, the change from downtown area to the Strip has tax implications (favorable to the Strip casinos). Mayor Goodman was always complaining about the Strip tax revenue out of his reach because it's in the unincorporated Paradise area.