Hm, well if she doesn’t look to chubby or big headed next to one I could always grind down the bottom of her dress, a layer less and she might suddenly fit in fine. Of course my girlfriend may never forgive me for such ungentlemanly behaviour with another woman, deskirting her, my my.
Well if your looking for a game system then you could look at savage world showdown, free and has a fairly robust vehicle construction section as well.
How are you planning on running it? One player does the zombies and the other the survivors, or more of a both players do hunters and zombies have their own routine. In either case I am curious to your plans, and would be delighted to look the rules over. (homemade games are a bit of a hobby for me, in their own right)