400 B block

If you are building with the sole purpose of biggest is best, then I believe 540 is the biggest you can get. But, that's a drag motor with a very short life span. The best B wedge package for all aroun life and power, is the 470. You can make the same power, a little up the rpm band (which is good, because sticking 600 pound feet aint easy on normal sized tires), and still have it last for years.