Pretty sure it was a 318. There's probably posts in the old Moparchat forum from about 20 years ago with him describing the build. He went by cuda66273 in that forum. That was one of the popular Mopar forums back in the day. He has a few videos of the car on his website FBO Ignition - Home of the HRR688. In the video linked below, it looks to me like an 11.21 dial in. While not 10's, pretty darn close.
I don't know the specifics of the car, email them and find out. Maybe its in the tuning book he sells. Light racecar, hot engine build, big stall converter and transbrake; i don't see why it couldn't be done. Most would likely put the money to build a hot engine into a larger cube block. But if you want to prove something with a 318, why couldn't it be done?