If that is truly a 67 motor and its never been apart then you should have the closed chamber 516's which are a good starting spot. They can make good power...especially in the 400 to 450 range...trust me because I am running them now!

Compression is the larger part of the battle for 383's becasue the aftermarket pistons are way down in the hole for compression. Don't use the guides available on manufactures websites to calculate compression.

If you go with KB's at least go with the 400's as opposed to the 162's.

Get a good cam in the 230+ w/ .470+ range and you're good to go. As you probably already know though the limiting factor will be the intake and exhaust. The stock carb will be taxed a bit too. To be honest 400 hp may be pushing it a bit...