The 500 is a 500; doesn't matter where it comes from. Unfortunately, it's incompatible with your car. Fortunately, it's not that hard to make it fit, and the rest of your drive-train will match. The blocks are so similar that it won't be a huge job to fabricate mounts.
The 455 isn't a direct bolt in. You can get it in but you'll have to get the exhaust done after because it wont line up. The tranny will bolt up to Caddy, Buick, Olds and Pontiac but putting an Olds in would not be as easy as a caddy motor. So maybe they meant they trannys interchange but not the motors.