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I would think the results would be best if the dash is removed. You will not be able to properly wrap the fabric and secure underneath for a clean install.
There is no way to upholster it with it int he truck, it would have to be sanded and wrapped, and the storage bins on top would not be conducive to upholstering, you would probably either have to fill them or design some trim for them, or the upholstery would not stay glued down, especially consider the magnified sunlight that area gets under the windshield.