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Mike S, Do you have the pics you mention in this post? I'm about to do the same thing. My idea is to use a plasma cutter to remove the "rectangle" outside the caster block area and remove OE "nuts" and weld in grade 8 M10x1.5 nuts then wire-feed the piece back in.
That's what I did once during a wreck repair.
Just weld a piece of like thickness sheet over you access hole when done.
Sketch out the nut on the outside before placing it and drill two 1/8" holes inside your nut trace to spot weld the nut.