There are two companies that make 4 bolt main blocks. Dart is one and i can't think of the other off the top of my head. Also, you could go with a 351 since those blocks are stronger.
Dart,SVO, and now World Products are making blocks.The 351W is also a stronger block from the factory,as pointed out by Joe.The 460 BB can also be a good option for a cheap race car looking for low et's but not much else (other that big smokey burn outs ).
Ok, I don't think I would want to go with a 351 or big block because of the weight it would put up front. Out of the 302 based blocks....which would be the cheaper route? The DSS block would be perfect, but I don't trust a stock block at all with 500+HP.