What spark plugs for 340

This is my story on Splitfire plugs. Back in the early to mid-90's I had a 1985 C4 Auto Corvette with Air Flow Research aluminum heads, Arizona Speed and Marine runners, extrude honed intake manifold, ported throttle body, TPIS cam kit, roller rockers etc... Nice 12 second car. Anyways I always used Bosch Platinum plugs, but everywhere you went you saw the ads for Splitfire plugs and how well they worked, and that they made more power etc... One day I was at the local Western Auto and there were the ads on the counter for the Splitfire plugs. I just decided to buy a set and put them in the car. I went home and in front of the garage, I took out the Bosch Platinum plugs, and just chucked them on the grass behind me, and installed the Splitfire's. Took the car out for a spin and I immediately knew that it was down on power, it was a noticeable difference. I went back home, retrieved the Bosch Platinum plugs from the grass, cleaned all the dirt and grass off them and put them back in the car. Threw the Splitfire plugs into the garbage. I later learned that Splitifre had a satisfaction guarantee and that I could have taken them back for a refund, but the garbage truck had already taken them...