If you find obvious clues like sticky crake and clutch pedals or long term rust out / water damage at the floor pan behind the firewall, look for pin holing in the roof or around the windshield gasket. This isn't a deal breaker if your present cab is good, because it can get you into the parts truck a lot cheaper. The overdrive four speed truck tunnel is the same as with an automatic but with a large cut out for the side shift with the plasstic boot and shifter adapter that attaches to the side of it. The top shift transmissions are different. But you may find a cab swap is your best option, and it makes pulling the engine a lot easier with the front clip pulled off.
