High Perf. Doesn't really tell you what the main bearing's are it was written on 2 and 4 bolt block's. A Mark IV is a excellent street or race motor , they weigh about 750 Lbs but can make ton's of torque. Even a 2 bolt will rev to 6500 R.P.M.'s they have an excellent oiling system . Good luck with your Rat motor. If your going to rebuild it you have to take the pan off anyway so you can check it then.