Although you say "14x12 brood" (singular) - I'm assuming you have two 14x12 brood boxes, and because you're asking for a loan of standard brood boxes, that suggests your Nucs are on Standard Brood (DNx) frames ?
If so - then why not simply transfer the DNx Nuc frames directly into the 14x12 boxes, and pad those boxes out with 14x12 frames ? If necessary (if we're talking April), make-up a dummy 14x12 frame for each box to use as a 'thermal curtain' to reduce the amount of box space available to the bees - or reduce the size of the box in some other way.
Then, as the season progresses, simply move the DNx frames gradually towards the outside until they're cleared, and then remove them one or two at a time, replacing them with 14x12 frames as you do so. Of course, the bees will draw 'wild' comb beneath the DNx frames, which is a slight nuisance, but it's easy enough to slice-off when you come to remove those frames.
It's by far the easiest way - if a little protracted.
LJ