Hmm... Older trucks sure don't leave a lot of room for adding things do they. At first thought that the angular section of the dash between the windshield vents and the forward vents would be a nice place for a 7" LCD, but after more thought, you would probably get a lot of reflection off the window and it would be hard to read. That flat spot on the top of your dash would probably be the best thought. Maybe have it be completely hidden when off, and pop up when it comes on... if your that resourceful
Your component selection should work well, but for the extra $20 - $30 or so, I'd go at least 512MB memory.