Looking for the right path!

I think, your goals are a little high considering the intended use. In terms of how I would do quick pricing: 400hp (1.17hp/inch) will run around $15/hp. 450hp (1.33hp/inch) will cost about $16.70 per horsepower. Something to remember is that these cars when new made an honest 270-290hp and were some of the most balanced street cars of their era. Simply by using modern performance components and modern high quality machining those numbers can easilly go up to 325-350hp with factory castings and manifolds. So as others have mentioned - shop around and don't get sucked into the "you need new heads", "need new rods", "need forged pistons" stuff. Those might help - but you don't "need" them to really make improvements and end up with a really great street package.