You can fit it in there, but you'll need to get custom headers because the 400M is taller than the 351 Cleveland. The motor mounts, oil pump and oil pan will work (from a 351C swap kit) and you'll need a transmission with a big block (429,460) bellhousing pattern like a C6 to get it to fit. An 8.8" rearend from a GT should be more than enough to stand up to the 400, but don't even mess with the stock 7.5" rear it's pretty much a waste of time. You'll also need a cowl hood or something along those lines because the M class engine (modified cleveland) was designed with a larger stroke in mind so it has a larger deck height than the Cleveland (351). Good luck, that sounds like an awesome engine swap!
------------------
'84 Mustang 5.0 LX
My car
[This message has been edited by 84stangLX (edited 07-20-2001).]