I've been playing around with a desktop dyno and if its at all accurate then you'll be hard pressed to make 500 hp with the Stealth heads, they just don't flow enough. Someone on this site did some mild porting on a set and when I plugged those #s in it really woke the motor up. However, I've been running this for motors in the 500 cube range. I'm in a similar position - just sold my shortblock and am planning the new one. I'm almost thinking you're better off to build a bulletproof bottom end on a 400 cube motor, slap on a set of Stealth heads, a "streetable" cam and carb and a big mother of a nitrous system. This way you can cruise all day long economically but have the punch when you need it. I've never been a big fan of NOS but I'm beginning to see the virtues!