Either he's made a mistake about the Autolite one barrel having a different bolt pattern than the slant six, or they've used a different flange pattern than probably 90% of the Autolite one barrels made, because that swap is something I've done first hand. I used an Autolite one barrel on a slant 6 in a forklift years ago and it bolted smack dab on and worked fine.
How is that possible? The mounting flange bolt spacing is different 2.9" on the slant and 2.7" on the Autolite carb. If you call Holley they will tell you it is not going to work. The guy in the Part 1 video even states it at 6:00 minutes in