The problem is not so much wheel size as what brakes are available
to fit/reach the rims. Brakes have a limited radial adjustment range
and ones made for 700c wheels on 700c frames will probably not
reach 27" rims on the same frame. Brakes used to be available in
several reach ranges, but these days are made for the standard
road bike which is 700c sized. You need to investigate what brake
availability is for your contemplated set up. In addition modern
brakes are made for modern frames which assume recessed nuts,
especially on the fork, and older frames assume external nuts which
leads to the mounting bolt not always being the right length. When
I rebuilt a Teledyne Titan ('75-76 era) I had to machine a new mounting
bolt for the front brake as the 'front brake' bolt was about right for the
rear brake mount.