I think you will be alright since you are going with a 400, but in some cases guys have had flywheel clearance problems with 460s and I think 351M bell housings. Some times they work fine and some times they don't, I have always wandered if the 351M and 400 bell housings are identical or if they are a little different. They do uses different clutch sizes and possibly flywheels.
-John