Will 400 heads fit on a 350 motor and if so what will the difference be in the motor and power?

Yes, 400 heads can fit on a 350 motor, but modifications may be necessary for proper alignment and clearance. The primary difference will be in the engine's compression ratio and airflow characteristics; 400 heads typically have larger valves and ports, which can improve performance. However, the overall increase in power will depend on the specific head design, camshaft, and other engine components. Tuning may also be required to optimize the engine's performance with the new heads.