To tell the truth, my knowledge of the pre-Crisis era of SHAZAM is sketchy, at best. Any pre-Crisis SHAZAM information is more than welcome.
All appearances of Captain Marvel published between 1939 and 1953 were published by Fawcett. During the late 1940's, Captain Marvel was even outselling DC's Superman. Tony LeBlanc, a Captain Marvel fan of some years, notes:
Stylized artwork separated the Fawcett version from its peers in the marketplace of the 1940s- early 1950s, by insisting that artists follow a style easily recognizable to the public (C.C. Beck and, to a broader extent, Jack Binder). Principal writer Otto Binder refined the adventures of the intrepid trio into an artform all its own.
With World War II in full swing, many Captain Marvel comics published during the 1940's expressed a strong anti-Nazi and anti-Japanese sentiment. This phenomena was probably typical of the popular culture of the time.
From 1953 to 1972, no SHAZAM comics were published, given that Fawcett had ceased trading.
In 1972, DC reached an agreement to publish Captain Marvel comics, but under the title SHAZAM (since Marvel Comics now owned the Captain Marvel trademark on their own character). At that point, it was deemed that all Captain Marvel stories up until then (including those published by Fawcett) were within DC continuity but set on Earth-S. That nearly explained why the stories should be considered "real" and why Cap had not encountered the rest of the DC universe. The 20 year gap between the last Fawcett SHAZAM issue and the first DC issue were apparently explained in a story in the first DC issue, where the Marvel family were trapped in suspendium for 20 years as part of an aborted plan by Sivana.
In 1976, in the pages of Justice League of America, Superman and Captain Marvel finally officially met. Fan interest of the era, plus the long-running court case between Fawcett and DC made this a much anticipated event. A DC story published earlier had Superman up against a character called "Captain Thunder", who was suspiciously similar to Captain Marvel, down to being a small boy magically transformed into a super-hero. (As an aside, in the original ashcan edition which unveiled Captain Marvel to the world, he was actually named "Captain Thunder".
As an aside, Captain Thunder is not the only character to be "inspired" by Captain Marvel. Big Bang Comics currently includes the adventures of two characters inspired by the Marvel Family: Mighty Man and Thunder Girl. The two characters are unrelated to each other but bear striking similarities to their SHAZAM counterparts.
1978 saw one another meeting between Superman and Captain Marvel. (They actually met many times during the pre-Crisis era). It was then that DC published a 72 page Collector's Edition comic pitting Superman against the World's Mightiest Mortal due to the machinations of a villain called "Karmang the Evil". I'm told that this issue was the first in which Supergirl and Mary Marvel met, also.