I test fit the outer wheel houses, got them where I was happy with the fit and marked the plug weld spots from the inner wheel house with a paint pen. Then I sanded the spots where I would be plug welding to and hit the spots with weld through primer. I sanded the entire wheel arch lip and hit it with weld through primer as well.
From under the bed, I clamped the wheel arch and drove a few sheetmetal screws in on the inner WH side to draw everything tight for plug welding from the top side.
Here is the top side after plug welding, you can see the sheet metal screws still poking through from the bottom side. I remove those in a bit.
here it is all welded from the bottom side. I drilled the wheel arch holes about every 2.5", clamped and plug welded from under the box.