Spark plug options for 18mm iron heads

I've got a 523" dove headed motor w/9.8:1 compression. I previously ran the NGK-WR5 plug, but now have a plate kit that I plan on running in the 250 shot range. The coldest that I've been able to come up with are an NGK-AP8FS which is an 8 heat range instead of the stock 5 heat range.

What other spark plug options do I have for the 18mm head?

What plugs have you guys ran in your old DOVE headed nitrous motors? Part #'s would be great!

Thanks,
Jon

I ran BF 22 "s in my old 427 FE on a 250 kit for years !!
Frank

I ran AR32 plugs with d0ves and an 82 jet