Endless Mike, they've fought pre crisis before.
Their biggest fight happpened in this big graphic novel called "DC Collectors Edition C-58: Superman vs Shazam", where both were mind controlled and bloodlusted, but were in full possession of their combat capabilities.
Basically, it came down to two things: 1. Captain Marvel was a little bit weaker, and got tired. His recuperative powers would've been much better in his home universe, if not inexhaustible.
2. Despite being at a disadvantage, Billy realized he could ko Superman with his magic lightning, and run away at super speed before he himself gets hit to avoid the transformation, and was about to do so when the wizard Shazam contacted him and insisted the fight must continue.
And of course, Superman promptly KOs him.
The thing is, the wizard claimed Billy weakens outside of his home universe like Superman weakens under a red sun, but he didn't actually say Cap is more powerful then Supes.. I'm wondering if maybe Supes vs Cap in Billy's universe would result in an endless stalemate. Cap gets endlessly revived by his universes magic, and Supes gets infinite energy from the sun...
If you factor in the magic lightning, that could go either way for Cap. Either he hits Supes with it and wins, or Superman has the presence of mind to grab onto Cap and pull him into the lightning, in which case he'd win, as he did against the Captain Marvel clone, Captain Thunder. (And that's more or less how Supermans won every single post crisis match, except for the first Eclipso battle..)
