I use the 817 which has a two bolt main cap, but I have the splayed four bolt main caps installed. The 817 is the strongest block of the three made, it has a higher nickel content and weights another 10 lbs. The 509 is good for about 600 hp. I'm not sure about the 511, but probably 500hp.

If you want to go for more than 750 hp, I would buy an after market block.