As much as the A/K RS-3 announced to the world that N scale could be taken seriously as a modeler's scale, the follow up Geeps signaled to the manufacturers that many of the compromises they had made to N gauge trains would no longer be acceptable to adherents to the scale. Demands for add-on detail, fine scale wheel treads and flanges and road specific details can be traced back directly to the demand for a scale length hood on the Atlas/Kato geep. Before this gaff, we took what the manufacturers gave us, and we liked it!
I bought several RS-3s back in those days, and still have a couple of them. But I never sprang for the GPs, because even in the magazine ads they just looked wrong. Also, when you consider that the old Roco GP9 was actually shorter than it should be, it just made the newer one look even worse. The NSN conversion was a feeble attempt, but it still didn't bring me on board.
If you go the route of putting an RS3 shell onto the mech, you can find older Kato engines on ebay all the time. The conversion kit for the body mount couplers is the MT 1152 (I'm looking at one right now). In fact, I think I have a surplus A/K RS3 in the drawer I'd be happy to make available to you for a modest price.
It will provide you with the proper side frames/trucks, and a shell that will fit without modification. Plus a drive full of spare parts should you need them.
Just let me know via PM
Lee