I forget the exact reasoning as to why Gerald Ford decided not to run, but Reagan had a lot to do with it. I remember reading that given the experience of 1976, where he had nearly lost, he was unsure if he actually beat him without being an incumbent; not to mention the polling was against him as well. He entertained the idea into March of 1980, but ultimately decided against it.
Without Reagan, Ford will jump right in with nothing really holding him back. The primary race would be interesting, however, given that many conservatives would flock to Phil Crane or John Connally, and Ford would have to compete with George Bush and John Anderson for their base; Howard Baker too. What must be considered if any of these candidates would back out if Ford were to jump in; among those, however, Baker is the only one I think who would endorse Ford, and even then it isn't a given.
So Ford is the heavy favorite, no question. However, it is dependent upon the early primaries. If Bush manages to still squeak a win in Iowa, you will see a lot of Ford supporters probably moving into the Bush camp; even if Bush loses for that matter, as long as it is narrow, he could consider it a "victory". Then you have New Hampshire, where Ford MUST win. Again, it is up in the air. Ford and Bush would be fighting it out, while Crane and Connally try their luck.
All these details aside, I believe that it would in the end become a Ford-Connally race, with Ford eventually rising to the top by the end; Connally would likely have won South Carolina, and then remained in the race till the end. Bush, however, if he had unbalanced Ford enough, could easily take the latter's place, especially with victories in Iowa and New Hampshire.
Again, on a closer look, it is really up in the air.