2 Barrel Manifold

Assuming a good carb, the 2bbl setup gives better driveability. But that good carb is a big assumption; it's grown difficult and expensive to find a really good carb (1bbl or 2bbl) for a Slant-6, so if your 1bbl setup is running well, maybe better to leave it be. Aside from the intake manifold you'd need a carb, an air cleaner, kickdown linkage complete if your car is automatic (See this thread for kickdown options), and if your '66 is an A-body (Dart/Valiant/Barracuda) without factory A/C, you'd need to convert from the rotating-rod throttle linkage to the cable-type linkage, which means a different accelerator pedal assembly. Alternatively, See the parallel 2bbl setup article. Photo documentation of a very well done such conversion is here.