I made an air dam for my van from two '87-'90 Dakota air dams. My van is an '87 but I think the '86-'93 bumpers are the same. Here is how I did it.
The air dams were literally pulled from Dakotas at LKQ. The bolts were too rusted to loosen so I pried it off with a screwdriver. Only the center is used and can be welded up later.
I used an oscillating multi-tool to cut the ends off the first air dam 26" from the center. The other air dam was cut 20" from the center on each side for the corners.
The pieces were clamped to the bumper to check fitment and mark the mounting holes.
There was a 1.25" gap between the bottom of the fenders and the corners of the air dam. From the ends of the first air dam, mark and cut 1.25" by 7.5" sections from the upper corner.
I clamped the pieces to a hood and welded them together with a cheap Harbor Freight 80w plastic welding kit. I used SS screen to strengthen the inside of the seams.
I then made some nut plates to bolt it to the bumper, it had three mounting holes. Clip nuts hold the ends to the fenders which I drilled for the bolts.
The ends stuck out past the wheel well so they had to be trimmed also. Not show quality but neither is the rest of the van.