There is some physics and ratio optimization that determine fuel economy. If both motors are big enough and designed with the same attention to efficiency for the job at hand and are coupled to appropriate transmissions, rear axles and tires, I think its reasonable to expect the smaller one will get better mileage.
Most cases where you hear a bigger motor got better mileage it really means the smaller motor was not really up to the job.
Based on what he said in another thread I don't think mileage is really something our young rebel friend is concerned about.
Unless you plan to spend serious money modifying either motor the 500 Cad (bored to 512) is going to make more usuable power than most other stock gas motors you'd find used for similar money. Of course there are motors out there that will outperform the Cadillac but not that you are likely to find cheap.
If cheap power is not important, start with something with more performance parts support. Once you start buying performance parts for the Cadillac you're going to wish you had a big block Chevy instead...
