The only reason I can see for the Allies to fight Franco is if he was actively hindering the WAllied war effort, sending ships to harass Allied shipping through the Strait of Gibraltar, or officially at war with them. Franco, of course, did neither; he was no fool, and he knew his country could barely stand after a sustained civil war. He gave Hitler a long list of requirements for Spain to enter the war, because that was what Spain needed to be able to fight. Hitler decided it wasn't worth it, and ignored Spain for the rest of the war.
So as it stands, it would basically be suicide for Franco to give the Allies an excuse.
If they did invade, though, it would be (as they said) a short war followed by a long insurgency. The Spanish army of the time had quite a few veteran soldiers, yes, but they had spent so much money, materiel, and blood winning the war they could barely fight a strong, mechanized army with full aerial and artillery support like the Americans. Furthermore, the WAllied forces would be rather elite themselves by the time Germany capitulated, having learned from grueling combat against the Wehrmacht.
That said, Spain is excellent protracted combat terrain. It's rough and hilly, especially in the north, and its people had been fighting for their lives since the 19th century. That creates a hardy breed of fighters. The Spanish veterans would have home field advantage, and would bleed the Allies for every inch, though they would sorely lack anti-armor capabilities and AAA.
However, one factor that should be remembered is that Franco's regime has quite a few enemies; liberals and Communists who didn't quite catch the first ship out when the Republicans were beaten. They would have gone underground, and many of them would be brutally purged over the coming decades. By 1945, however, there'd still be a sizable amount of them left, and they would gladly help the WAllies take down the Generalissimo. The Communist diehards would be a more wildcard factor, equally willing to help and hinder the WAllies to advance their own agendas.