Thanks for posting a link to our page, Ron. You're such a good neighbor.
BB Panel-
I can see that this car has our Battle Cruiser adjustable ladder bars and the econo coilover kit. (The frame is not ours.) The best thing to do in this situation would be to cut out the rear rails and do an entirely new backhalf. If that is not an options, here's a couple things to consider.
These bars most likely measure 32" from axle centerline to the front rod end bolt. Putting a 4-link in this frame would mean welding in a new crossmember closer to the rear which isn't impossible but I would be concerned about frame rail clearance.
Check out
THIS BUILD ARTICLE for details on how this might be accomplished. I know it shows a pickup but if you're trying to work with what you have, it's essentially the same process and the components are the same.