[amcmike]

Any 2004R trans is going to require a heavy duty high performance build, so any core will work. The valve body on the GNs are better, but whoever builds your trans should include the valve body upgrades in the build anyway. Find the cheapest core you can, so it can be gone through thoroughly. There are rumors a Chevy-only bolt pattern exists (as opposed to the dual pattern BOP and Chevy). I actually have never come across one, but you should check to make sure before you buy. Obviously if you source a core from a BOP engine, it should come automatically with the correct pattern.

FYI, when you talk to a builder be sure to inform them what engine you're using. Many rate their trans by hp, but it's really torque that limits durability. So putting it behind a 450hp Turbo Buick V6 is a whole different animal than putting it behind a 450hp Pontiac torque monster.