13.00 - good run! Sounds ike you're hooked and if you have a 9.0 sec Chevelle... you're a wiz under the hood. Glad to have you here.
There are P+P services, heads, cams, intakes, headers, exhaust, supercharger and turbos (5.7L) that have been attempted with various and debatable degress of success here in this forum. We even have a member who is in the process of tubbing his Magnum,
You'll find ots of good info on what to try and what not to try.
You'll be interested in Project Morpheus - software to help us get the most out of our mods. Not yet available... but it's going to be ready soon.
Welcome and enjoy!