Spark plug help NGK Plugs

Hi guys, I am trying to find the hottest plug that NGK makes for some Magnum style closed chambered RHS Indy heads. I currently run FR5s with gas, but I am switching to E85 and was told by Mark Sullens that I need a hotter plug. I believe I may have found the hottest plug, FR2B-D. It has the same reach and thread as the Fr5's but I wanted to ask for some more help before I buy them. Thanks!