Yes, I already said your order was right from looking at the pics. Yes, a big gap can prevent starting but .011 should get a fire or two. We don't know why you have that plug? What plug you asked for. Makes me wonder if your other parts are correct for 1966 GMC 305 V6. Parts counter guys or internet application charts probably never heard of that motor.
Find TDC at compression and check rotor. I do it with a remote starter switch (jump the starter switch) and center plug on dizzy removed.

Quote from Pertronix for Ignitor II:
In stock applications, the manufacturer's recommended spark plug, and
spark plug gap will work best. For performance applications, the spark gap
may be increased up to 0.010” from manufacturer’s specifications to take
advantage of the extra energy produced by the Flame-Thrower II coil.
Since PerTronix cannot test every configuration, the end user must
determine what spark plug gap works best for their application.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn