Flak Cannon, no, but I think a sawed off shot gun would give a similar effect with the wide area of effect plus kick to blast your way through

My only problem with ranged weapons is running out of ammo.. if it takes approximately 3 well aimed shots to take down a zombie, you would need 3000 bullets assuming you landed all your shots perfectly. Unfortunately I see this as improbable, why would you carry around 3000 bullets in the back of your truck?

I'd opt for a light weight melee weapon like a katana. At first I considered a battle sickle, until I realized I would have to be way too close to the zombies for comfort.

As for a flamethrower, I'd be concerned that it wouldn't actually stop them and make them much more effective as now you would be fighting flaming zombies instead of the regular non-burning ones

I like the tank idea.. just run them all over