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The sheet metal is an integral part of the bus construction. Be sure to have the bus on level ground and remove the sheet metal by drilling out the spot welds. Only do one window at a time to reduce the chance of compromising structural integrity.
While the sheet metal is off, either spray foam your insulation back in or use the thermal insulation and add sheet insulation. Then replace the sheet metal using spot weld method or rivets.
Another method is to not remove the sheets at all except to remove the insulation bags that are already there through a small opeining, and then spray foam insulation in through cut out 1 inch holes.
If you are still not satisfied, then consult with other bus conversion enthusiasts or even boat builders, as they always have good ideas.