No, the pistons do not definitely mean you have a 428. It could be a 390 bored to 4.16" I had one of those once, and a cylinder wall cracked after a short time.
Usually the 390 blocks are taking to the stock 428 bore of 4.13", but that's not a good idea, in general, either.
Look for a "428" cast in the water passage under the center freeze plug.
paulie