How many miles are on the engine. Was it just assembeled with poor gaskets or is it tires is whay it's leaking? Point is, just a gasket job and freshen the heads will typically show any weak link in the rest of the motor. It's a lot of work to tear it apart. Not freshening the bottom end also is a lot of wasted effort unless it's a 15000-20000 mile motor.
Doing a backyard port job is a good way to screw up a decent set of heads. If you don't know what your doing and don't have a flow bench it can be a lot of wasted effort, or worse the demise of a OK set of heads. Without a compression change and cam change it may show SERIOUSLY small returns in power.
But... if you must...
http://www.small-block-chevy.com/images/port1.jpg
http://www.small-block-chevy.com/images/port2.jpg
http://www.small-block-chevy.com/images/port3.jpg
http://www.small-block-chevy.com/images/port4.jpg
Just remember these drawings are for some of the more popular performance heads and you need to verify casting thickness before you blindly sit down with a bunch of tootsie rolls and an air grinder and and start hogging out material.
And wear eye protection and a DUST MASK. The nostrils and the lungs really don't like the fine metalic shards a carbide burr and/or a tootsierolls send into the air.