Other contributors will be surprised to know that it will fit because the 400 is actually a small block engine, same as the 289, 302, and 351 (W). To tell the difference between a small and big block look at the spark plugs.
In the small block the plugs will all have the same angle (point the same direction) but the big block the first two plugs will point in one direction and the last two will point another direction.
Look at the 400 you have and you will see it is a small block. (BTW, FoMoCo really stretched the small block to the limits with the 400 so it cannot be bored.)
If your Mustang has a small block V-8 then it will drop in without much fuss. You may have to change the exhaust manifolds, no problem just take the manifolds from the Mustang engine and bolt them to the 400.
You might need to custom make some motor/transmission mounts. also the drive shaft might need to be resized.
I think there will be enough room ok for it to physically fit, just a little custom work to get it in.