Will a Broan 750 fit in the same cutout as a NuTone 8664RP?
I haven't worked with these exact units but they should have pretty similar requirements. I was able to find a diagram on the Broan website for the 750 that allowed me to extrapolate the size of the cutout.

Bear in mind there's a bit of margin for error in this trick but as you can see, the diameter of the cutout would be roughly 13 1/2&quot;. You may have to ream out your drywall a bit but that's better than having a hole that's too big. And remember your grill is 15&quot; so you'll have aprx. 3/4&quot; of fudge room all around. In all likelihood the drywall cutout won't be an issue, but you'll probably have to modify the blocking to accept the new unit, which (as long as your framing is as it should be) won't be too much hassle.