In my opinion, keep the original 327 original. If you want more HP, buy a Crate Engine already re-worked and modify it more if you choose to. Keep the 327 in storage as someday you might just be glad you did. If you end up never using it, you will then have something pretty valuable to sell. But that particular engine will only be original to your particular car once.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America