I did not mean to offend you.The 400 uses the siamesed cylinders,there is not a lot of material between the cylinders,they also use a short rod 5.565 rod.I have built quite a few of these over the years.The advice is use very good head gaskets,and a 5.7 or longer rod.Don't get really greedy with n20.I have used 175 shot before on the crank actually about 15-20 bottles and the stock crank actually hung in there.I would also upgrade to a larger cooling system.