Unlike many, the E450 has a drain plug at the bottom of the gas tank. The EFI pressure test valve is near the transmission dipstick. Green cap, as I recall. Just watch out. If the system IS pumping, that'll be around 50-PSI so you want to protect your eyes and not allow any ignition sources.
Fuel filter requires a 5/16" disconnect tool If you replace it, and it's the large diameter one, be sure it has the long "necks" that go into the connectors on the fuel lines. Test for this with your disconnect tool. If you can tell you wouldn't be able to get it into the connector, the next time you'll have to cut the filter necks off. This is because Ford USED to use a different connector, where a locking clevis went in from the side.
