Does the motor turn, and if so does it sound like it's draggin? Which 400, Olds or SBC? First I would make sure you didn't get a wire crimped between the block and the tranny when you installed. Fairly common occurence, if all your wiring looks like it's supposed to I would do as thesals suggested and check the torque converter next. That will bind everything up and keep it from runnin. You may even have the correct converter in but not have it seated in the pump correctly. Whenever you swap automatics make sure that the torque converter moves freely after you tighten the bell housing bolts. The converter should have free play in it. If it doesn't then you either have the wrong converter or it's not installed into the pump properly. Those are the 2 places I would start. Keep us informed.