Thanks, Andrew; this is the information I need. The only hope now is that the forward, upper tab on the racks can reach and fit onto the boss on the arch already installed on the bike -- it serves as a forward fender support, and also supports the headlamp (wires run inside one leg from dyno hub to lamp).
I have asked the seller if he will kindly give me measurements between mounting points.
All of this as a possible alternative to the custom lowriders build for the frame --which bolt to fork leg and arch, but were foolishly made without any dropout area attachments.