There is no specific number to where the 302 block splits. It was rumored 500hp. The rumor came from back when I started doing this. Hardly anybody did a computer tune back then. There was no where near as much understanding about tuning, and RPM, and how that stressed the block. There is a guy on sloppy that has put over 800 to the tire on a Turbo Mustang; complete stock bottom end; consistently. His name is not coming to my head. I know he is from South Carolina. I am not trying to say that if you through a ton of power at a 302 block it would be fine; more that the quality of the parts, and the time, effort, and restraint put into the combo/tune will be more rewarding and less likely to fail in the end.
Keep in mind that the primary reason a factory 351 block is regarded as "stronger," is due to the casting design in the main webs that allows it to be drilled for 4 bolt main caps. If you leave the 2 bolt main caps on there, it's not much stronger than a 302 block. So add the cost of three 4 bolt main caps, and the machine work to the 351 block when comparing prices. I still contend that it is usually cheaper to get a 4 bolt main 302 block into a Mustang than a stock 351 block.
FYI, my last 347 put 372hp to the tire with 192cc heads. You had to wind the snot out of it before the power kicked in. I don't think my new 347 will do a whole lot better.
Kurt