Lloyd, my best guess would be how the rear of the frame is shaped. The down-tube that attaches to the SPF frame (as viewed from the trunk side) is at a steeper angle on the SPF. The CSX, Kirkham and ERA (and maybe a few others...Autocraft, B&B?) most closely resemble the down-tube rake of the originals, and it might have to do with how the frame is in the back.
If there was one visual pet peeve that I have on my SPF, it would be this one little detail. Then again, I'm usually the only one in any impromptu gathering that's aware of it.
Modifying is a lot of surgery and not worth the hassle. You could also end up weakening the structural integrity of the existing design if you elongated that down-tube and a different angle and still tried to tie it in at the frame points.
-Dean