I'm going to guess if you are installing this yourself then you have some experience in where the "normal" spots to pass the wire is. Which gauge wire are you running?
This may or may not help...but I had an 86 Ranger where it was actually easier to go out under the king cab rear section, where a body bolt grommet was and run it under the truck to the front.
Otherwise, if its that tight at the firewall...i'd get my own rubber grommet and drill it.
If in doubt...HIT THE GAS!! It may not always be the right decision, but it will end the suspense!