Easiest mods that give a noticeable gain would be a low restriction air cleaner and an advance curve kit for the single 4 bbl distributor.
Dual quads look great but a single 4 bbl works just as well if not better. The stock manifold can have the plenum opened up and a larger AFB used, or find a '66 Q-jet intake and build a Q-jet for it.
The intake plenum mod is referred to as 'Doc's mod' .....it consists of opening up the 4-hole carb opening to 2 larger ones by removing the divider between front and back barrels.
Consensus on headers is they don't make much more power and dealing with low ground clearance and fitting them around the stock starter are more of a downside.
Replacing weak valve springs helps rpm.
A new cam or high-ratio roller rocker arms (Tom's RR) help breathing too!
One thing you don't want to do is rebuild it with pistons with less compression and then use thick head gaskets to lower compression even more!
