The most common area for rust is above the windshield. The roof structure is double skinned and makes a great condensation/moisture trap. If the roof structure is still solid, drill some access holes in the inner skin and treat it with something like Eastwood's.
The next worse area is rocker panels. This is another enclosed space that wasn't treated well by the factory, however the factory did switch to galvanized metal, in the 80s but they can still rust as the zinc is worn off or consumed (galvanic corrosion).
Frames on these trucks don't usually crack unless severely fatigued. Cowl areas can crack. This could occur due to body flex. If there is a crack in the cowl area, suspect the frame too. Many old frames that were worked hard will have worn out or missing rivets which will allow the frame to flex a lot and some of that flex is transferred to the cab. The common rivets to wear or disappear are those that connect cross members to frame rails.
Ed
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