I'd say at least 400 ponys as long as it is in good shape. The question is more about "How long will it last" to me. There are guys running C4's behind built 351C's at the strip but they are highly modified. A stocker IMHO will take a 400 HP street blast every once in a while without grenading. The same trans might not last a day at the strip making run after run. With a good shift kit and an external trans cooler the same trans would likely last a long time.