Plug help
 

I was told by my local shop that I can no longer get the AC Delco R43XLS and the R42 was the replacement but I am not convinced. I could not find anything about it on there web site.

I'm running the GM performance aluminum ZZ502 heads.

Does anyone know if this is true or do i need to cross to a dxiff plug. I like the AC's better than the NGK but have not tried any others.