Re: where to start?
Not knowing just how fast you want to go, the 327 you have can be made to perform very well. You may not make 400 hp but, you'll be close enough to have fun with the car. Look at the 302 in the First Gen cars. They were screamers from the word "GO." While they didn't have the torque of the big inch motors, they were more than "cool" when it came to hearing that little motor rev to the Redline.
The comments about big cubic inch motors are obiviously correct but, you'll have to do some things to the car to help control that power/torque (i.e. subframe connectors, some form of traction setup, etc.) The rearend you choose will also have to be well thought out. The GM 10 and 12 bolt can handle the power of the big blocks but are, on the low end of cost, better suited for the small block putting out a moderate amount of power. My dad had a '67 Coupe with a mild 454 ci in it. That car would haul 'em, when the pinion yoke didn't break.
400 hp with the 327 is stretching it, unless of course, you shoot the juice. Even then.... It all comes down to money. How much do you have? How much do you want to spend? How fast do you want to go? How often are you going to go fast?
I'm probably in the minority on this matter but, I believe building a car, any car, with the intent of staying original is good, if you have an exceptional base to start with. It sounds like you have that part covered. But, in reality, how many of us have the money it takes to restore one to showroom condition? I don't get the impression that you are trying to build a Barrett Jackson record breaker. The bottom line is, it's your car. Do with it as you please and have fun while doing it.