What heat range??

I have an 88 Wagoneer Limited w/ 4.0, I recently swapped the 88 longblock engine for a 98 longblock engine. I kept all of the fuel, electronics, etc. from the 88, just the block, head, etc. is from the 98. My question is which spark plug heat range do I use? I know that the 88 takes champion RC9YC and the 98 takes RC12LYC. Please HELP!

I am using the head from the 98. I thought that was the case (same head same plugs) but I just wanted to be sure thanks!