A stock 302 block, properly machined and inspected for cracks and core shift, should be absolutely safe to 400 horsepower. Using a main stud girdle, you should be safe to 450 horsepower.
Many people have used the stock block for higher horsepower applications, sometimes much higher. But there is a real risk of main web failure above these levels. When they do fail, it is catastropic; usually the entire engine is lost.
Stock blocks are available new for around $325. Ford Racing Performance Products offers a 302 Sportsman block for about $875 that will handle 200-300 more horsepower. They also have a full racing block for about $1650 that will handle 1000+ horsepower.
The single biggest factor in block longevity is avoiding detonation.
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