[cobrashoch]

The Blue Thunders are a mid to high end range head. The CJ heads are a high RPM head (all 3 versions) and will make more high end power. In a 2500 pound car like a cobra I doubt you will see any differance between the two. I wouldn't recomend porting the B.T. Chevy port head on a 520 though. Just pocket port "em" and clean them up by smoothing out the ports and keep the compression down to about 9 to 1. That combo will get you to around 600 honest horsepower and on pump gas to boot. Any more power than that will require some big time "beefing up" and will make your car heavier and be very costly. I'm using the stock port Chevy B.T.'s on a 545 and a supercharger and the bottom end charge is unbelievable in ANY gear. In short, if you go bigger in cubes don't go wilder with your parts. Sage advice you won't regret. I also recomend you use the Chevy rod setup if you can afford the the extra cost of the crank too.
cobrashock