Snowtoy has given alot of info.  I would look at the frame beside the fuel tank.  And the crossmember in front of the tank too.  Common areas for rust. 

Sit in some cabs, and see what fits for you.  I'm the opposite of Snowtoy.  I fit better in a 2nd gen standard cab than in the 3rd gen standard cab.   I have way more leg room.