175lb/ft. I know you'll find different numbers across the web. It's 175lb/ft. Since it's going to be a single-shot, and you won't be drilling a gas port, it really doesn't matter what steps he takes in installing it. He's going to have to trial-and-error the headspace, though. Have fun taking that extension off and putting it back on a few times, while machining the extension for the proper headspace. I don't know how many AR barrels he works on, but he's going to need a specific barrel extension installation tool to pull this off.
Chambering from the 7.62x63 down to 7.62x51 is going to be quite the chore - I'd like to hear how he's going to do this.
Good luck. This neva been dun befo...