I will be doing the same thing to my 07 F350. I got luckly and found a complete spare bezel that goes right from the drivers door to the pass side air bag. I bought it cheap and will use it to fabricate some kind of a mount right onto the back of it. I just ordered the Lilliput 8" wide screen and expect it to show up this week coming.
I will just figure it out as I go how to fabricate a mount onto the back of the bezel. To help me choose a screen, I printed out in actual size on thick paper all of the different models, cut them out, then fit them to the back of the bezel. The 8" wide screen appears like it will fit with minor mods.
So I guess my suggestion is to remove the dash bezel, and start looking at obstructions on the back, taking measurements etc.
Other than that, if a true double DIN will fit, you could buy the 7" screen mounted in the 2xDIN brackets and just bolt it up.