Yeah i got a pair of 6x9 pioneers for a gift and i couldnt return em, so i had to find a way to stuff em in there. Thats when i found out that the stock speakers were 6x8 , so i had to drill a few holes and do some cutting but they fit now.
The speakers are good old Ford 6X8's. I didn't have to take out the back seat in my 97 Super Cab to remove the rear speakers. There's enough room to remove the speaker behind the driver by just popping off the interior trim, remove a few screws and the seat belt loop up top. That made plenty of room to get to the speaker.
Depth should not be a problem for the rear speakers. Any 5X7 or 6X8's should do.