I would not use a lapped joint on a cab corner (or pretty much anywhere else, for that matter.)
A butt joint is much easier to get the panel and the original metal edges lined up. And you can deal with shrinkage of the welds.You may wind up with small valley along the weld line on a cab corner, but that's not hard to deal with.
Take a look at MP&C's excellent writeup on MIG welding sheet metal HERE

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.