Short answer, not really. Long answer:
The coupler mounting would have to be redone--they mount inboard of the pilot which will severely limit the side-to-side motion when the pilot is fixed. Even if you can get away with that minimal amount of swing, the coupler on the power truck is not even mounted to the pilot, but to the power truck itself, which of course swivels. Supposing you sourced a spare dummy truck pilot, so you have two pilot mounted couplers: their mounting position on the pilot is such that, were you to separate the pilot (with its coupler mount) from the truck and pin it to the frame, it would foul the truck and prevent it from rotating on the curves. Look closely at a photo of the underside of a GP and you will see what I mean. And once you move the mounting, you will have to re-engineer the self-centering feature, too.
Once the coupler is figured out, the pilot itself presents more problems: It will need a lot of material added to it: merely cutting it off the truck and sticking it to the frame will make for a "high water pants" effect fully as offensive as the gap you are eliminating.
And on top of all that, it is not even a GP pilot!!! MPC used the power truck and pilot from the postwar 44-tonner.
And once you get past that, you still have the stamped-steel handrails, the molded-in grab irons, the opaque screens, the cab-full-o-motor ...
The MPC GP is a toy train, and it looks like a toy train. It does happen to be scale in size, but it is every inch a toy. I say, love it for what it is, or leave it alone. (Or send it to me, where it will be loved and pampered 
)
