Got a little more done Thursday evening.  I don’t know what this goop was – if it were seam sealer or something else, but there was goop in the grooves that needed to be removed:

So I used my angle grinder with a wire wheel attached:

and it took that goop right off.  NOTE: Make sure your wearing a hat and eye protection as this junk flies everywhere!

I removed some seam sealer from the side so I could see what I needed to line up against for proper fitment:

Checked for better fitment again:

Cleaned up the edges a little more:

Now for some more clean-up, final cutting, and to weld it in place…  I need to change the wire in my welder from the thicker .030 I used in the jig to something more suitable to use on the sheet metal.