Hummm... .
An interesting idea. The owners manual makes a big deal about not exceeding something like 120# (#???) of permanent front mounted snow plow equipment. And if you move them far enough back, the WXD effect will magnify the result. I also suspect these batteries weigh more than 40# each. All of this makes me think the result might make some difference even on these very heavy trucks. Since they make winch set-ups for plugging into the rear receiver, there are some applications that use the monster cables to move high amperage loads the length of the vehicle.
Cons? Wow, the cables really would need to be huge to be sure you got full voltage for cranking. You'd have to be careful with routing and supporting them to avoid shorts, due to either vibration, or wrecks. There might be more vibration damage to a battery mounted in the bed, especially if you go all the way back to the tailgate. Acid leaks would get on your "stuff" instead of dripping on the front fender well.
It would be a bit of a gamble; you'd do a lot of work, and then the results may or may not be satisfying to you. I see this as one I would most likely never get around to doing, but, I do think the idea has some merit.