I think when you mount your pickup into the Bigsby bridge plate and place the pickup in the cavity that will pretty much locate the bridge for you. I'm sure you'll have a little movement Longitudinal and lateral directions. Measuring 25.5 inches from the nut can help locate it in the neck direction. To locate in the other direction you could use strings and weights like I did here.
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It would be a little more difficult for you since the bridge would be loose. Once the bridge plate and bridge are installed, installation of the tremolo unit should be pretty straight forward.
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Jack's Disclaimer: When I say something.... always ask yourself ..... "What the hell does he know?" _
Guys - learn to disable the flash on your digital cameras.