The driveshaft length is the same as long as your donor truck has the same wheelbase length.
The ECM is different and will throw codes when it doesn't see the torque converter and 3-4 shift solenoids. It will otherwise work fine. If you use your auto trans harness, you won't have the wiring for a clutch switch but you can add that part by wiring the auto trans neutral safety switch wiring to the neutral switch on the trans. The truck will only start with the trans in neutral. The only way around that is to use a dash harness from a manual trans truck.