This is a no brainer for me, go with the 500 Cadillac. You can take a stock 500 and bore it 30 over with 10/1 comprssion. Then pick a good cam, get an edlbrock intake with an edlbrock thunder carb. This cheap and easy build will out perform most small block chevy motors and will only weigh 60 more pounds. If you do not mind spending the money to put a set of worked aluninum heads on that 500 Caddy you will make more power from idle to 3000RPM then any pump gas small block could ever muster. 500 Cadillac motors are all over Ebay and they sell for next to nothing. Without buying the heads you can build the motor I am talking about for 1000.00 and that includes machine work. Make sure you polish the combustion chamber of your heads to get rid of hotspots, go with an HEI distributer, and find the stamped pulleys. The factory cast pully is a heavy turd that should be used for an anchor.