There have been several threads in the past week or 2 about switching from a 350 to a 400. You can search back to see some of the responses. For the crossmember, you can, depending on year, either move back and drill new holes, get the right one, or cut the mounting tab off the existing member, and weld it back on the front. Jake explained which years would work with moving the crossmember back in one of those posts about a week ago. Good luck.
If a man says something in the forest, and there is no woman there to hear him, is he still wrong??