McBTC

If I'm getting it, that's the diameters of 700 and 650B wheels respectively. So, the difference between the radius of the two is 3/4" or, 19.05mm. Accordingly, if using my plastic rule from college days, if the rear brake pad is at the top of the stop on the 105 calipers' arms on my Z85, I could swap out the 700 wheel (that cannot be fitted with 28 mm tires due to frame-impingement problems) for a 650B (11 spd compatible hub) and the rim brakes would still work if the bottom of the stop on the brake's caliper arm is within ~20 mm. Then, I could have a road bike that'd accommodate 28mm or larger rear tires. But alas... arms look to be a mm or two short.

mrv 

650B (584 mm) Conversions for Road Bikes

you might have to switch to long reach brakes. I have a lovely set of Tektros on a bike.
https://www.rivbike.com/collections/...tro-r559-allen like these.

I usually go off the Bead Seat Diameter for the rim, then measure how far the brake bridge is from the center of the drop outs.
https://www.sheldonbrown.com/tire-sizing.html
650b = 584mm BSD
700c = 622mm BSD
So your 700C 311mm (half the BSD). You want to go to 650b, 292mm, so you need another 19mm of reach. (I just took another way to get where you are - so we all think we're right. I think.)

The Tektros listed above range from 55mm to 73mm reach. Kind of depends on where you are now with the brakes you have. Not sure if you wanted to buy brakes with your new wheels.

Some other things to consider (problems I've run into after changing from 27in wheels to 700c wheels):
- My fatter 700c tire hit the bottom of the brake, so I could only run a 32mm tire, not 37mm I wanted ~ now I'm trying MAFAC Racers to get more fatter
- Smaller wheel means lower pedals. So be careful cornering. Ideally your fatter tire will get you back where you belong. Hopefully.
Other than that, no problems. I think.

McBTC

Good recap and options... I'd like to keep my bike and have a 28 on the back but the more expensive it gets to accomplish that the cheaper it becomes to put the "Z" back in OEM condition and go for a quick sale at ~$450 ('15 at ~1,700-1800 new) or so and just buy a new alloy (endurance/'gravel') bike with 105 components for <$2K (e.g., perhaps a '18 Domane on sale).