Basic 318 Mods

Headers, timing curve and higher initial timing, hotter coil.
leave the carburetor for last... even then I wouldn't mess with changing it out till I had some better valve springs and something like a 340 cam.
Toss a cheap cut tq convertor in it, 3.55 or 3.91 gears even before that.

Simple.
If you want real power increases beyond the 20 hp range... address the heads.