You will have to cut the tunnel and enlarge it basically a total re-do - also you will need to build a new cross member for the 400 - if you are using an SFI approved case will get you away from blankets and shields that's one plus as between the bell housing and case you are covered - its a lot of work with no gain other than parts availability - powerglide would be easier. If you are having problems at your power level - get a new transmission builder