If the rocker panels are gone you will have to put some new ones on.
The dash does not matter. It is different. I would use your old dash and transplant all of the wiring from the old cab to the new one. I doubt it would be compatible.
Changing the cab means removing the nose, then taking the cab off and putting the new cab on. Then replace the nose. The bumper will also need to come off to get the nose off. You might prefer to take the bed off instead but you risk chipping the fenders if you get the cab just a bit to far forward.
1997 3500 club cab Automatic 3.54 gears towing and camper package. Still stock with 116k on it so far. 4" dynomax stainless exhaust, 4kgsk, and pacbrake exhaust brake waiting for installation. Still need ideas for improvements for hauling 15k fifth wheel.