Even if you could reverse direction on a squirell cage fan that would not be a good thing to do... all the fins are designed to throw air OUT....and suck into the hub... that would NOT be good to try....
Even if it is a puller it would completely mess up your ram air effect.
Why do you need ram air ? Why not plan on running the fan all the time you run the truck ?
By ' ac fans' if you are talking squirrel cage.. they pull to their inside and blow out the side.
When dealing with fans be advised that you can not simply reverse the fan blade on the fan shaft. Still moves air the same direction... LOL.. just in case you were thinking of trying it....