Do 351 cleveland heads on a 400m engine add more horsepower?

Yes, 351 Cleveland heads can enhance the horsepower of a 400M engine, but the extent of the gain depends on various factors, including the specific head design and how well they are matched to the engine's overall setup. The Cleveland heads typically have larger ports and valves compared to the stock 400M heads, which can improve airflow and combustion efficiency. However, proper tuning and compatibility with other engine components are crucial to realize the maximum potential gains.