If I knew what specifically was included in the "upgraded fuel system," I'd be better able to answer your question. In general, small block Chevys tend to foul the # 6 plug before any of the others. This is a result of the uneven fuel distribution in the intake manifold. It also can be provoked if the EGR valve is sticking partly open, or if the EGR controls have been modified or bypassed. These motors also will foul plugs if the valve stem oil seals go bad, if there is an intake manifold leak or a ruptured vacuum diaphragm on a vacuum driven accessory, if the exhaust air injection system has been disconnected, or if a low restriction exhaust system has been installed (particularly if the catalytic converter has been removed), without recalibrating the fuel system metering or ignition advance curve to match the new running characteristics. Some aftermarket ignition systems can also create plug fouling (particularly if a coil is used which does not match the coil driver module, or if a strictly racing system is used on a street vehicle, or if the plug gap is not set to at least .045" when using a MSD CDI). I often find that this motor is unusually demanding on the brand of plug used. The plug brand with the best fouling resistance in SBC motors is Autolite. Bosch platinum or Fusion is also a great option; but I would not use Bosch Super non-platinum plugs in this motor. NGKs are a second choice. It sometimes becomes necessary to use a hotter plug heat range; or to change between a non-projected and a projected tip plug, in order to control fouling.
But if all the plugs are black; the fuel mixture at low speeds is way too rich. And that usually comes from either a defective fuel pressure regulator, or a vacuum leak in the hoses to the MAP sensor, or ignition timing set too far retarded; or engine or exhaust modifications which were not compensated or retuned for.