I have a 454 with Dart Iron Eagle heads. I was running AC Delco MR43T's (.035 Gap) and blew the center of the plug out. After checking with my engine builder he told me I was using he wrong plugs and I should now use AutoLite AR3935's at (.025 Gap). Will I notice any difference in performance?

The BBC 308 Dart IE' s take a plug with a flat seat and washer. Don't the MR series AC plugs use tapered seat? If so, you need to switch types. Both are available in any heat range. For what it is work, we use the Autolite Racing 3911. This heat range should be real close in marine application for a 9.0-9.5 CR BBC.