If you want to get creative you could always cut a larger rectangular hole in the bottom of the cab and then use two 4 ohm speakers wired in series side by side. Wiring them in series makes the two effectively 8 ohm. If you have access to a junk yard, most cars use 4 ohm from the factory and the door speakers are usually 6 inch. Chrysler still uses 2 ohm, so don't use em unless you want to wire up 4 of them in series.