The way to do this is as follows:
With bridge constructed, and the sills or supporting structure in place, place the bridge where it will sit, as it will sit. It should not rock or tilt.
Then, with one or two lengths of correctly connected flex track, lay the track over the bridge so that the ends of the curve extend at least six inches beyond the abutments. This way, you are making the determining factor the bridge's curve, and not making the bridge conform to what's on either side of it.
You use joiners or nails to align the ends of the bridge rail perfectly with the oncoming tracks on either side of the bridge.
Now you'll see what, if anything, you need to do to alter the footing of the bridge. Maybe your radius is going to have to be manhandled a bit somewhere to get it to lie more-or-less centered on the deck. But, if you built the bridge according to a design, with the correct radius for the curve, and you situated the footings and abutments correctly, you should have nothing to do but to remove the joiners, lift the tracks on the bridge, smear some contact cement or wood glue lightly over most of the deck ties, and then relay the tracks. They should fit well, and a light weight over them will press them nicely into the adhesive for a few hours until it's dry.
I'm not clear on what you propose to do, but it sounds complicated, fraught with opportunities to make a mess, and to possibly mess us the appearance, or even to break something.