I have to agree with a few people. I know that a 400 and 403 are a lot different, but as far as the siamesed thing goes, it holds true. Chances are the fault is in the heads, gasket or the block. I personally ran a standard fel-pro gasket with RTV black near the water ports and had no problems. And as for 425 HP-HA! pick up a manual, and run a comparison to a stock 400 block. Add about 10-15% and youll be there.