I can make a number of suggestions; but I don't know what you have to work with in terms of resources (such as used car parts, or engine builders that can turn out Chevy motors), or your budget, or the restrictions on what you can do to the car imposed by the European Union and your local government in terms of emissions etc.
In sunny Florida you can put a tag on the back of a Top Fueler and drive it in on the street because the restrictions they wrote on noisy exhausts are unenforceable, no one checks emissions or safety equipment with the exception if it has a windshield it has to have at least one operating wiper and one tail light and head light if operated after dark. I ran a thousand horse monster on the street until I retired it because it was starting to scare me with how squirrely it handled.
Some places in the US are a little more restrictive and where the principle cause of a lot of the problems that forced the death knell of muscle cars. I won't mention any names, but the other 49 states know who to blame.
If it were mine I would pull the original motor and transmission and replace the 327 small block with a SBC 406 built to crank out about 400 to 450 horsepower, going through a 200R4 transmission to a set of 3.90 to 3.73 rear gears. The advantage is it looks like a 327 if they have a visual inspection. Properly tuned it should also pass the sniffer test if they do a tail pipe sniff. The OD allows you to run the steeper gears and still get better mileage than the 327.