Your best bet is to phone Ford with your VIN or visit a local dealer. They'll be able to print out the original factory specs of the van and it shouldn't cost you a thing!
225-hp, 5.4-liter V-8 (natural gas)
255-hp, 5.4-liter V-8 (petrol)
As NG isn't as efficient as gasoline, you lose HP/torque and mileage (1-3mpg; city/hwy). I'm not sure if it'll work with the E series or not, but index
makes home NG filling stations!