I assume you're using the original generator.
1. If you want to keep the car as original as possible, see if your local generator/starter shop can increase the amperage.
Be sure you have the highest CCA battery possible and that it's fully charged prior to a parade.
Consider replacing your beacon with a #17-D or -B. You'll save a few amps; plus, the four-bulb models weren't introduced until 1958.
Stick with the #28, as it takes a little less juice than a Q.
2. If reliability trumps originality (at least under the hood), I would suggest swapping the generator for the highest amperage alternator you can get as well as a battery - or better yet, two - with the highest CCAs available.
Whichever direction you take, use the siren sparingly at low speeds.