Ok, I'm going to post this one last time. RPM is a killer on the 351/400M's and also on the earlier 400 engines. You can build a healthy stump pulling engine with those but you have to respect the fact that they can not take rpm above max 5,000 rpm. Yeah, you can make it breathe better and all the old school hotrodding tricks give excellent results. The problem is that rpm above 5,000 rpm sets up a harmonic vibration that causes the webbing in the block to flex. This web movement causes the cam bearings to start breaking apart resulting in a slow drop in oil pressure. Eventually you have so little oil pressure that the engine suffers failure of the connecting rod bearings along with valve train wear. My heart felt advice is if you modify those engines also invest in an ignition system that has the ability to control high rpm. I personally prefer MSD ignition systems that have the rev limiter. You can also kill those engines by downshifting the transmission in a situation that results in way too high rpm. Don't forget this.