As I also posted on your FaceBook page, the real answer is Yes and No... the inner fenders are made of two pieces - the outer part which attaches to the inside of the fender and is outside of the wheel and tire (let's call that the OUTER inner fender), and the inner part which attaches to the stub-frame rail and is inside of the wheel and tire (let's call that the INNER inner fender).
The INNER inner fender is the same and will interchange just fine between a 65 Fury and a 66 Monaco, I believe. The OUTER inner fender is model specific, and I do not believe your 66 Monaco OUTER inner fender will fit your 65 Fury's fender.
The other caveat is that the Fury and the Monaco are of different wheelbases, and there may be a very slight difference between the two that's based in the stub-frame, but I think it's mostly in the rear of the car and where the rear of the body shell kicks up over the back axle.