First, I see no reason for your outlined proceedure not to work. It does seem rather costly and labor intensive. For all of your effort, you will only loose a small percentage of the weight.
My suggestion would be to sculpt a new body design, one that is far more aero than your original, out of foam panels. Cover them with fiberglass. You can use all of the same mounting attachments and hardware glassed into your new body.
You'll achieve about the same weight reduction, save yourself a lot of work, have a more aero vehicle(aero counts, even in town), plus have a one off vehicle, unlike any other.
