Agreed, as with most cars , look for rust in the wheel wells, behind the wheels on the fenders and 1/4's in the trunk & floors... all the typical areas. But as mentioned, the rockers are the most importaint part as they are structural in the verts.
Check the top opperation and frame. Check how the body panels line up and fit. Look for any previous major damage inside and out.
Verts will have an "X" pan underneith in the middle of the floor pan, typically vibration dampners or "Cocktail Shakers" at each corner (large cylinders bolted in) you can see the ones in the trunk corners easily.
The 400's should have a trunk 400 emblem, the 400 hood with emblems, the Arrow emblem on the nose of the front bumper, and chrome trim in the center of each grill (Hockey Sticks) TH400 tranny etc. As mentioned these are all bolt on's.
Hope it works out... if not... there are many more out there! So don't jump at it if your unsure!