Hi, I have a 454 Magnum carbed with CMI headers running in fresh water.

What plugs should I use??

I know alot of people like the AC Delco & NGK. NGK BR6FS,AC MR43T

And I've also seen Iridium BR6FIX 2318 0.04, but that might be overkill??

I personally like the NGK (BR6FS). Had a AC plug come apart once have never used them again. My 2 Cents

AC MR43TS or the NGK equivalent.
They have a slightly extended tip and are better for cold starts and idling, but are the same heat range.

Found a great link re: heat range, and how to choose if you need to change.

http://www.ngksparkplugs.ca/products...ugs.cfm#marine

eg. chip & headers one heat range lower

MR 43TS are no longer available, the only ones similar are R43TS (the M is for Marine, is what they told me)

You can use the regular R43TS. The M is for marine and the only difference is that the marine plugs have more corrosion resistance on the body.
The other choice is AC Rapid Fire #1's. The body is the same as marine plugs. They do cost quite a bit more.

Merc manual also list Champion RV15YC4 as an option. I've been using those for a while now, and no issues at all. As a bonus, they are plated, so have shown less external corrosion than the Delco R43TS. Easier to find in my neck of the woods also.