Typical bridge ties were 8"x8"x8' with 7" spacing between them. Regular flextrack, even if the tie spacing is adjusted, won't look right because they aren't thick enough. I recommend handlaying of wooden ties of the correct dimensions or use special bridge-tie flex track such as offered by ME.
If you want to use regular flextrack, employ a ballasted deck over the bridge. You won't have to make any adjustments to the regular flex track except to add the inner guard rails. Besides, this is the typical construction method on contemporary US railroads.
Mark
