How do you replace a cab corner on a 1984 chev truck?

Place the replacement cab corner tightly over your cab corner on truck and trace/scribe the outline on truck. Remove replacement corner. Cut along scribed line and removed old cab corner. Use a "stepping" tool or a pair of pliers and step the first 1/8 inch or so of the cut edge for the new panel to rest on. When fitting the replacement corner OVER the existing cab corner, it should have traced about 1/8 smaller then actual size because of being atop the original. Wire/MIG welding the replacement panel is best option. Weld small tack welds along the replacement corner about 4 inches apart. Keep alternating weld tacks to prevent heat warpage. Complete welding and grind smooth and add body filler (Bondo if needed) prime and then paint. The replacement corner can also be held on with "panel adhesive" as an alternative to welding. Some swear by it some don't. I have used it before and it works well for certain applications, but "I" would weld your replacement panel in place. Most all rental stations rent small welders cheaply if you don't own your own.