[VintageMusclecar]

Bart is right, headers (and a free-flowing exhaust system aft) will open the door for significant power improvement, esp. since they allow you to step away from the OE cam should you be so inclined. If you plan on staying with the factory manifolds, stick with the GM cam as it's been proven time and time again to work best with them. Quality machine work during the rebuild can also make a noteable difference. A little port work on the heads can pay off quite nicely if you're not against the idea.

Aside from that, stick with the tuning basics re: jetting, timing, lash, etc.

Eric