I read in here somewhere that the limit for a 351 is about 600-700, and after that it is in danger of breaking. How come I see cars in magazines with more than that then? I was at my friends house the other day and on the front page was a chevelle(i think) 350 with 1036 hp...

 

That limit is probably for the stock internals as in the crank. Because the crank is cast it can come apart with high HP numbers, if you get forged internals i.e. crank, rods, pistons it should be fine as long as you have it machined for 4 bolt mains. Other than that there might be "special service" blocks that are thicker, kinda like the Ford 302 sportsman block.
-P.

 

The "limit" is based on the block, the stock crank, rods, and pistons. RPM's kill the rotating assembly. To go further you have to upgrade. Do not have a production block machined for 4-bolt mains. It will actually weaken the block. You can take a sportsman 351 block and upgrade it to 4 bolt mains the right amount of material is there for it. I am not sure about the 302 sportsman block. Once you go past 700 rwhp with ny chevy, ford, mopar small block the stock internals become an issue.