I believe that they started with a single sided swing arm (mono-shock)...so a G/S, or ST swingarm would do as a start point...
Then the box section second side was fabricated and welded onto the original swingarm.
I could very possibly be wrong about any part of this, or maybe ALL of it.
Personally, if this was for my project, then I’d be speaking nicely to Mikeyboy at Overland and classic....(and saving lots of money over what HPN would charge you, even if they would sell you one)....mind, I could be wrong again, and Mike could charge you the same as HPN .....!
Keep us updated if you go for it...and PICTURES too please.