To make an appreciable difference in compression, you'd need to mill the heads ALOT. In other words, too much. The 400 has well under 8.1 compression stock. It blueprints in the 7.6 range. To get it to 8.1 you'd need to mill about .060" off the heads. See where I'm goin here? IF you're just gonna stick with the bottom end, either find a good matched camshaft and leave the heads alone or run a small shot of spray on it. It'll take it as long as the system does not run lean. Those stock pistons are a little better than people give them credit for. Don't believe it? Call NOS. They'll tell you. I've seen some of these rustang guys sprayin stock 302 bottom ends season after season with no damage. but they got their ducks in a row. you could always think about a turbo.....