The problem with this is that it is pure ASB. Gulliani would never get the Republican nomination. Never ever ever. He's too associated with New York/the East Coast, too 'elite'-seeming, too socially liberal, and he has a Nelson Rockefeller-style record in his personal life. He is absolutely anathema to the Republican base in almost every sense, and he's not hugely convincing even with independent voters.
Combine that with the fact that the first primary states are socially conservative, (to various degrees) rural or sparsely populated, and the simple fact that they are primaries (higher 'base' turnout) and Gulliani is simply never going to get any traction under any circumstances. Even if you removed McCain from history, you would probably simply end up with Huckabee (or a moderate who runs in the place of McCain - Romney? Tom Ridge?) as the nominee.
If we do ASB him into the top slot, then I can't see how he would be anything other than a weaker candidate than McCain. The base stays at home, Obama gets his vote out and takes enough independents/undecideds to win comfortably.
The simple fact is, Gulliani is about fifty years too late as a Republican Presidential candidate.