425 hp is decent goal. Now that you know about the benefit of closed chamber versus open chamber you can put a plan together to work. Assume you get the heads your left to pistons and cam really. You won't need anything exotic as far as the valvetrain really. The stock rockers should work fine for 425 hp.

I guess a major limiting factor in terms of stock appearance would be carb, exhaust, and intake. I know the F.A.S.T. guys are runniing stock appearing stuff, but they usually are stroked out to the max which you indicated you're not able to do. With that in mind I am not so sure you'll be able to meet you goals with the stock manifolds and carb. You may need to make some compromises there.

You could run an aftermarket intake and carb. With a painted intake it is hard to tell at a glance and no one really sees the carb...headers are a dead giveaway though.