do you just wanna run it parallel, or do you want to run an isolator? what ya gonna use it for? winch, lights, sterio?
you can make or get a custom tray and put them right next to each other. i think f series diesels had two batts from the factory, you could hit the local yards and see if you could find the drivers side tray to mount the 2nd batt in. then move your coolent overflow tank somewhere else.
fireguy50 has a good write up on this i beleive. check out his superford page i think hes got pics.