I'd say go bb. Also, I like EFI too...... better low end and mileage (and yes, you can tune to HP too). going with the 454 will leave you a lot of options for massive hp and torque.

also, build a tranny, rearend, and susp to handle the 400+ you're planning. I KNOW a good BB can easily put out that much power. breaking 1000 is hard, but 500 is nothing nowadays. Not being a chevy guy myself, I can't suggest much other than plan, plan, plan, then build.

~Critter