So, this sage continues. The 26.4 seatpost seems too large. So I made a shim for now from a aluminum can and put the smaller post back on.

Then I noticed just how much room there was around the 700 x 23 tires. Not sure why I didn't notice that before. I put my one remaining Continental GP4000s II 700 x 28 on the back wheel and it fit with a done of room to go. It's measuring 29.27mm on the Open Pro rim at 100 psi. I'll actually run it at 80 psi though. But there's easily room to go much larger on this and still clear the brakes, seat tube, chainstays and the chainstay bridge. I bet this frame was designed for 27" wheels originally.

A quick snap of the space on the rear now with the 700 x 28 on it. Heck, this alone is reason enough to keep it.