There should be a picture of how the belt should go on your engine on the front of the engine compartment. Does the new belt number match the belt that you removed? When you stretch out the old belt is it exactly the same length as the new belt? If all of these things match up properly, then you must relieve the tensioner as you are installing the belt. The tensioner usually has a place where you can tighten or loosen the tension on the belt in order to get the belt on the car.
First draw a diagram of the existing belt routing. The serpentine belt can then be removed and installed by a person of average strength by pulling hard toward the front of the vehicle to clear the belt off of the alternator pulley. Install by routing over all pulleys except the alternator (a long screwdriver or serpentine belt tool helps with the lower pulleys) then over the small alternator pulley last. I have done this job several times without removing the wheel or splash shield, and it can be done with NO tools on the side of the road. The tensioner pulley spring is really not very strong. Good luck.
The tensioner pulley is the small one on the bottom closest to the passenger compartment. I think it is a 13MM nut on the pulley that allows you to crank on, in order to release the tension so you can string the belt. Ford put a plastic dirt shield right in a spot that gets into your way when you try to find where the pulley is/where to get the wrench onto it, so I ripped it off... Anyhow, this is a two man job, because you need someone to release the tension laying under the van, while the person topside strings the belt onto the pulleys... You need to take off the front passenger side wheel too, in order to give yourself room to work....
Next time try pulling on the belt hard with both hands toward the front of the vehicle. It will clear the alternator belt with no tools, as the tensioner spring is fairly weak on these. Good luck.