I need a little feedback on what heat range plug to run. Specs are:460 with dove heads, 10.5: 1 , mild hyd roller cam 278/286 adv., and duraspark. I currently use motorcrafts, but the plugs the book calls for using duraspark are for the later smaller plug heads.They seem to look clean, but i have burned up two factory coils in 5 years. I currently have a jacobs coil and motorsports wires. thanks

 

If you are burning up coils, I'd doubt that it is a plug heat range problem.
You need to check the voltage going to the coil while the engine is runneing(red wire). It should be around 8 to 10 volts. It should only be 12v when cranking the engine. If you are getting over 12 while running, you may need to add a ballast resister to the red coil wire.

 

The duraspark coil is not supposed to have 12v ?I know the points setup is that way, but was unsure about the duraspark. On a ch*vy your hei gets a continuous 12v.

 

No, this is why they have a crank and a run wire. The red w/black wire should be the start wire.