ground strap
 

Where is the ground strap or straps located on a 91 .e-150 4.9 van

It is always easier to remove the air cleaner assy then you can see more.

You just remove the two 8mm bolts up front and loosen the hose clamps on the large hoses that go to the throttle body unclip the PCV filter and remove the entire air cleaner assy . Very easy to do and much easier to see what you might be looking for.
I believe the battery ground cable connects to the engine down near the starter mounting, not sure, been a while. Battery is on passenger side so easy to follow cable.
Also, if you can remove the doghouse cover inside the vehicle, that makes things easier too. If you don't have NEVER SEIZE to put on the floor bolts just use vasoline or grease and that makes them much easier to remove later, if necessary.
If you cannot find a 10 gauge wire from the engine to the sheet metal, just use what you have (14 gauge or larger diameter) and connect one or two and they will do the job.
From my experience with those engines and the Econolines, they were very dependable and with good maintenance provided very good service.

Always a good idea to inspect the radius rod bushings on those because they gave no sign of problems until it became a big job to replace the mountings.
Keep a close eye on the front brake pads because they give no warning when worn and will destroy the rotors.
For what it is worth, I found Ford brake pads outlasted all other pads and were worth the extra cost

Thanks