The Nagant wouldn't be a bad choice. It would take the adrenaline for a zombie horde (or charging Germans) to not feel the kick. And the butt has a nice piece of steel for clubbing after the ammo runs out.

I like the idea of a truck gun, but being an insurance agent I've handled too many theft claims from cars to feel OK just leaving one there 24/7. If I was going to leave on it would be a Nagant or SKS.

You could also buy the cannon from the for sale forum and just tow it around with you everywhere you go.