[Richard]

There are a number of things to consider in this project. Maybe you are aware. First, welding galvanized can make you sick. Second, you cant make long runs at a time with the welder .Short stitch welds with plenty of cooling time OR you will melt the floor above the patch. Third, and I understand why, you are building in the reverse order that it was originally built, so sandwiching anything in the layer where the osb was subjects it to the heat of the weld.

Just an idea. Bring the scab to the lower edge of the framework. Predrill some holes in the patch, and use door and window frame foam to fill the space above the patch applied through the drilled holes.