Stock is .35 but since you have aftermarket ignition I assume coil is hotter as well so i would open them up to .45. Use copper core since they are more conductive and spark easier and perhaps index as well. You should consider getting the old factory "fuelie" heads and ram's horn ex manifolds.
The 1966 Chevy Impala ignition switch can be removed by loosening the retaining ring at the top of the ignition switch. Remove the wiring harness. Reverse the process to install the new ignition switch.
Check for fuel (bad fuel pump or clogged fuel filter) Check for spark (bad or weak ignition coil)
Dwell 28-32 degrees Spark plug .035
The 1966 Ford Mustang 200 cubic inch straight six spark plug gap is .032 to .036 inch
The spark plug gap for a 1966 Mercury Marauder 410 6.7L V8 is .034 to .035 inches. The new spark plugs will not need adjusting, however, as they come preset.
Points Gap: .014-.016 (26-28 degrees dwell) Plug gap .035 Auto-lite BF42 plugs
On a 1966 Ford Mustang : 200 cubic inch inline six cylinder engine : The spark plugs are gapped at ( .032 to .036 inch )
According to one of Fords websites : For a 1966 Ford Mustang , 4.7 liter / 289 cubic inch V8 engine : ( the spark plugs are gapped at .035 inch )
A 66 Mustang regardless of engine doesn't have an ignition module from the factory. It uses breaker points inside of the distributor housing to directly control the ignition coil.
Could be a bad ignition switch....
Get a chiltons repair manual from the library for your car.
.32 to .36