LMC cab corners are a higher quality( thicker metal and better fit).
you could have a shop weld on a patch and you do the finish work 300$+
you could buy a grinder from harbor frieght, buy the patch and buy some 3M 8115 panel bond adhesive and cut and step your patch and (Glue) it on, glue isnt a good term for that adhesive, it hold toyotas and GMs bedsides on. 200$+
or buy seal out and soak the inside of the cab corner and por15 the outside until your ready to deal with it.
theres a brace under that spot that might be slighty swiss cheesed
hope that helps lol
edit. thats an exteded cab? it should all be one piece from LMC, if not your looking for rear rocker section also then, i dont have a LMC book near me