June 1967, close your eyes  for a moment and imagine Percy Sledge singing "When A Man Loves A Woman" and filter that with Bach's "Sleeper's Awake" and you have...

A good initial idea of what the first classical classic-rock song sounds like that made it to the American Top 40.

Vocalist Gary Booker (organist) sounding like Ray Charles somewhat obviously didn't hurt, and the lyrics yes pretty obscure, probably more so than a thousand "American Pies".

Procol Harum, "A Whiter Shade Of Pale" recorded May 1967: