I would say, go for a small block ford. A 351W is good. It will fit easily into any small block car, and you can stroke it to get big block power, with small block weight, space, less modifications. There are also good aftermarket parts available for the 351w being that most 5.0 parts fit 351w blocks. If you were to go big block, parts are generally more expensive. Aftermarket Aluminum Small block cylinder heads are much more expensive for big blocks than small blocks. Also, make sure that you teach your son the dangers of street racing, because all teenagers love to drive fast (me included). A 500 plus horsepower car for a teenager takes a lot of responcibility. I have seen kids and fast cars make some bad decisions because they make poor decisions with the amount of power. Sorry about the safety part, but new drivers are the most at risk, and nobody wants to ruin a badass car anyway. Just my .02