Wow, sounds like someone tried to construct their own "minivelo" without knowing what they were doing!
The 650B mockup makes the wheelbase appear quite long, I'll see how that will affect the handling.
Sub-10" BB height (250mm) sounded low for me too, but I just read from Dave Moulton's blog about his 1953 TT machine having only 9.25 inches (23.5cm)! So I guess, with the shorter cranks and watching out at tight corners I should do fine.
I'm really intrigued with the rusty NOS frame pictured above despite sloped top tubes not being exactly my thing, aesthetically. The seller told me that even the virtual top tube would be under 50mm, and that the frame should clear 28x650c tires, no wider - so no hope for 40mm 26" I guess?