I am assuming you want to go from 27” wheels to 700c wheels, right? That’s only an 8 mm change in diameter, and you are going to more than make that up by running a fatter tire. So you are not going to crunch your crank. As to brakes- most likely your centerpull brakes already enough adjustment range to accommodate the 4mm height difference in the brake pads.
Anyways- measure the dropout spacing. It’s probably around 126mm. Here is a 126mm freewheel hub wheel:
https://velomine.com/products/wheel-...26mm-qr-36h-dw

And post a pic of this NOS mixte please. Sounds like an amazing find.