You may need an adapter plate to mate the engine to the transmission. PAW (Performance Automotive Warehouse) sells the adapter plate. You may be lucky enough to have the dual pattern bellhousing. You can tell those ones from the others because of the extra bellhousing bolt holes. I would check first before trying to lower the engine in place and finding out the hard way.
Other than that just remeber that the 400 is externally balanced so make sure you have the externally balanced, balancer and flywheel combo.