mid 80's monte ss clip on 83 regal estate wagon

The body line at the bottom of the fender is the reverse of the same line on the Regal.

Someone else may chime in about any differences in the curve of the leading edge of the door vs the rear of the fender.

Can it be mounted...sure...will it fit correctly...no, but will be unique.

I would also check the alignment and gaps between the fenders and doors. Are you talking about the entire front clip, fenders and all?
I recall that there was a European guy on G-Body Forum that was putting a Monte SS header panel on a Cutlass Cruiser wagon. He realized that he needed the fenders and hood as well. But I think he had many hours of metal work ahead of him. Never saw any more progress of his build.
I guess if there's a will, there's a way, as they say.