I'm not familiar with Autocraft. Do you man Autolite? If so, the 605 is a pretty standard copper core plug, with a #3 heat range. That plug is used in a lot of stock late model engines, and has a slightly shorter reach than you would expect (by 1mm) for a SBF.
As you increase the performance and power output, you usually decrease the heat range. Usually, 1 heat range per 75-100 hp.
But, getting back to the original question, what do the plugs look like when fouled? Are they
oil or fuel fouled? A picture would be nice. I'd also like to see the back of the strap.
Also, what gap are you using? I'm not sure what static compression that engine has, but the gap should probably be something like .035"'ish.