I made some more progress today. I was hoping to use the Ellaspeed seat pan on this project, but it's not available, at least not now. So I'm going to go forward with my plan B. If in the future the Ellaspeed kit is available, and I hate the result of my plan B, then I'll swap it out. Plan B is free...so that's a good place to start. So what is plan B you ask?
Plan B is removing the vertical section of the DRZ seat and removing most of the stock rear fender.
Here is the seat with a notch cut out

And the remaining section.

As you can see, this seat was in rough condition, so I have nothing to lose by cutting it up.
Here is the bike with the seat in position and the rear fender (not yet modified). The seat still needs to be shaped and I need to get some aluminum "T" sections to created side fairings to hide the sub-frame and integrate the seat & fender.

With the tank installed and the head pipe in place, I had an opportunity to look at my options for the radiators. In several articles I read about the inspiration bike, Leo Yip mentioned that fitting the radiators was the hardest part of the build. With this in mind I have been planning several alternative methods to attach a radiator. It turns out the KXF450 single radiator would fit if I mounted it to the front of the frame. I know this because I built an virtual radiator out of cardboard.
But after confirming that the KXF unit would fit, I decided to see what was so out of whack that Leo had to relocate the left and right radiators.

As you can see in the photo above, the tube/hose that connects the left and right sides of the radiator (by passing through the frame) wants to go through the front of the fuel tank. Other than that - they fit. They clear the lower edge of the tank and the exhaust header pipe.
So I started thinking - why not just bring the radiators to a shop and have the inner connections rotated so the are vertical like the outer one?

I'll bring the radiators to a race shop I know and see if this is possible. If so, what I was expecting to be hard may just be quite easy.