Playing around some more. First I tried to use to different 130mm rear wheels. I didn't really have to exert much to spread the stays but neither one would seat into the drive side dropout. Then I went and grabbed two different 126mm rears and both went into that dropout with no problem. The narrower wheels had a modern Bontrager R3 700 x 23 tire and it cleared the seatpost ok. Then I put one of the old GP 3000s 700 x 23 tires on a wider rimmed wheel it it clears OK too. I mean there's not a lot of room at the seatpost with either tire but it's rideable. As a reference a Q-Tip can easily pass between the seatpost and tire even in the tightest area due to that crimp. And to be honest if I remove the dropout adjuster I'd get another 1/4" of clearance easily. Otherwise there's plenty of room under the brake bridge and between the sides of the tires and the chain stays. A plump Continental Ultra Sport on the same wide rime clears the front fork with a ton of space. So....I may be able to steal the beautiful Superbe/Arc En Ciel tubular wheels off the Opus III for this. It's running 700 x 25 Veloflex Roubaix tubulars that may just clear the tight rear just fine.

Some pics trying to show the clearance.: