The bumper bolts at the lower duct are only two of about a total of 7-8 so I personally think you'll be fine. The important ones are the brackets on the side of the bumper that is located under the front guards as they align the bumper straight.
Because I've done this myself (and have not finished!) I've always had the bumper attached at the bracket and held on by only two or three bolts as I just couldn't be bothered screwing all of them back inThey're holding up fine (since October last year haha). Then again I'm pretty half-assed most of the time.
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