There isn't any '77 Monaco/Fury out there for GTA IV, there's just 71 Fury and 74 Monaco, that are totally different cars
Myself I prefer the Fury, are almost identical, but I had some time ago a experience with one and since then I'm in love with that car, besides it really looks badass
But a police version of the '74 Monaco is also a good idea, CPD used a lot of them, and NYPD used '75 Gran Fury, that similar model than the '74 Monaco...