Mark's right on the money. A competent shop will not do anything to the flexplate when balancing in order to maintain that standard. Then if you need to replace the flexplate down the road, you can replace it with a standard 454 flexplate without worry of affecting the balance. A flywheel for a 454 has that same amount of weight in the exact same place on the flywheel as it is on the flexplate. From a balance standpoint it does not matter how much weight you add as long as the weight you add is weighted correctly. What I would do is compare your flexplate to a stock 454 flexplate (that you know has not been modified). If they are the same, you can go with a standard 454 flywheel. You can also go with a 350 flywheel (less mass) by adding a balance weight.
See this link
http://www.speedwaymotors.com/p/3262...ing-Plate.html
Good luck, Dan