What spark plug to use

[65k10]

Howdy,

trying to dial in my new(ish) 383. Engine was built a couple years ago and has seen little use but now truck is together and ready to become the daily driver. Engine builder may have taken a few shortcuts at the end and not sure if these are the best plugs to use (may have been what was laying around the shop!) I've sorted out all the other gremilins and this is the last thing to figure out. Engine is 9.5 to 1, pump gas, Brodix heads and intake, full length hooker headers and 2.5 inch exhaust, Edlebrock 1406 600cfm, mild cam, and HEI with 8.5mm MSD wires.

The plugs in it are NGK R5671A-8 non-resistor. It idles a little erratic but runs like a scalded dog when you punch it. Got more power than I expected which is not a bad problem to have. I have had a couple plugs go bad, not foul out, but just go bad, so no I'm thinking maybe these plugs aren't the best choice.

Any advice is appreciated.