What would it take to get a streetable 400-450 horse 360? The engine is a 76 so I don't think it's a steel crank, right? Also, how much power is a factory sure grip rear end good for?
Streetable is something that is only really defined by what you can put up with. If you don't mind steeper rear end gearing, and slippery stall convertors then it's really easy. If you need power brakes, high rear end gears, and low cruise rpm then 400hp gets a little harder and 450hp gets a lot more difficult.
You still need the basics - good heads, good cam, and solid lower end. Then you need the rest of the driveline to match and deliver the goal. So - what's the goal? What is streetable for you? What are you doing with the car?