Originally Posted by Charles Wahl

I would take careful measurements of a "Professional" frame to determine whether it's a good "fit" for 650B conversion. Your PX-10 had short enough natural brake reach (I know, having one myself) that you could have used Raids or even Weinmann 750 calipers mounted in the usual way, but in my experience that's unusual. English bikes from the 70s often have pretty long reach for the rear brake -- though I have a 1974-ish Holdsworth Professional frame that has shorter reach on the rear than the front -- go figure -- but too short a reach may be an issue if you want to mount fenders. Then there's the maximum tire width that you are able to fit between the rear chainstays (can be dealt with to some extent by dimpling, but the same Holdsworth frame here will only has 30 mm clearance between stays for a 700C wheel) and at the fork crown. Your PX-10 had plenty of clearance in the rear, but a racing frame won't. It looks like the fork crown and blades have pretty good clearance, but be sure. Fork rake (usually slight on a racer) has already been mentioned. Then there's bottom bracket (i.e. pedal) clearance, since you'll be dropping it 23 mm by going from 27" to 650B -- well maybe not that much, since you'll make some up if using wider tires.
Just sayin'. I'm interested to see what you do with this one.