Our large block Vanguard engines come with a Champion RC12YC spark plug.
Although the gap can vary from .030 to .040 on new motors, they should be at .035. We have found that the AP3923 spark plug is a good upgrade. It is an Autolite Platinum plug and comes in AP and APP -the later being double platinum and lasts longer. It is a slightly cooler plug and although the details regarding why we use this are long, we like to use this plug on performance motors and/or when replacement time comes.
Don’t underestimate the value of having fresh and well performing spark plugs. For starting, acceleration and top speed – good plugs, produce good results. They are the heart of your engine.