Whats the difference between a regular cab 4x4 91.5 ton dually frame and an extended cab 4x4 92 3/4 frame. both are five speeds if that helps. which one is heavier built i guess is my question.

 

They should be the same cross-section, just different lengths.

 

The frame area under the cab is different between a Club Cab and a Reg. cab. The Club cab has a longer drop to compensate for the longer cab. Also the Cab mounts and such will be in the wrong location.

 

Actually the drop area under the cab is the same on a club cab and a reg cab. The difference is the length behind the drop. The rear floor of a club cab is higher. So you could put a club cab on a reg cab frame. Cut 4 inches off of the back of the frame and make a club cab short bed. You would have to move the rear cab mounts back also.