I did one with a mig and it turned out fine. These bumpers are for looks mostly. They dont need to be tigged. But I would weld it. Not use glue. Not to mention that the glue could have a chemical reaction with the body work or paint.
First I ground the paint off everthing. Then I loosely bolted everthing together, and moved the valance forward to match up with the bumper. To create a smaller gap. Tightened the bolts and then tacked the two tog
Then I placed a piece of 1/8" cold rolled steel in the gap, tacking on both sides as I went.
Then I blended that all out. You could barely feel the gap. As a small flat spot. Then I addressed the sides. This truck is a 87 which has a shorter bumper, so I marked the wheel well on the bumper. Cut it and plated it in.
Go slow and dont let it get too hot. Mine turned out real nice. Hope this was of some help.