Most 500's will run on 87 octane gasoline. They are 8.2 Liters and for $2000 you can get a good rebuild with more power. Just opt for higher compression pistons and a bigger cam. It might be cheaper than a 454 because the initial price of a 454 is higher. Plus you will ruin the cad with a Chevy motor IMHO. Chevies are boring building different engines is more interesing.
If you plan on making more than 500hp then the Chevy motor is better because of the extensive aftermarket.
http://cad500parts.com/head/head.htm