Not sure but I thought I heard that the block needed to be clearance to run a 4.5 stroke. No proble making 600 hp but it might be hard to do with iron heads. Be much better off going alum. It all comes down to money. The more you spend, the faster you go, the more hp you get. Of course the money you spend on the hp might not end up being the only money you will spend. Once you start making that kind of power other "weakest link" pieces with show themselves. Then there are things like the $1K that you might have to spend on the fuel system upgrade to feed all those new ponies. Hey, nothing's free man no matter what they say on the Velocity channel.