Fill in the gap at the front and grind down. My welder is on setting C so I get a full penetrated weld even if the pieces are butted tight together. This means I have to bump the wire speed up a little to prevent burn thru. This also means tack it and move to another spot and tack it letting it all cool down to prevent warpage. I would love to learn how to TIG weld. Maybe some day. Doing it this way adds a lot of grinding, but the results are a full penetrated solid piece, that cleans up nice and smooth when ground down. It is a MIG with solid core and a bottle. I think it was burning the primer off the back side.
I will grind down any high spots on the back side before I primer it back there. I plan on shooting raptor liner in the wheel houses so this splice n dice wont ever be visible from either side once done.