I recently bought a custom run Wheeling & Lake Erie SD40-2 from Pat's Trains to convert to 2-rail with scale wheels, scale couplers, fixed pilots and full-length handrails. The plan is to run this locomotive under DCC on 2-rail track. I have other MTH locomotives that I run on DCC but they were already equipped with scale wheels.
The pilots are from a Lionel Wheeling & Lake Erie SD40-2, I didn't like the MTH pilots. Lionel did not have pilots with a plow so at some point I may modify the MTH plow to fit. To secure the pilots to the underside, the notches on top of the pilots needed to be ground off. I accomplished this with a diamond chip cutting disk on the end of a rotary tool and a metal file. Each pilot took about 2 hours to prep. The pilots are secured to the shell, not the frame, with 3M very high bond double-sided tape. The frame is slightly recessed in the shell so the shell was the best option. No spacer was needed since the pilot fits perfect once it's smoothed over. Lionel scale coupler mounts were used along with Kadee 745's. No shimming was needed.
The handrails going to the pilots are from a 2-rail Atlas SD40 that I had laying around for other projects. I wasn't thrilled with the look of the long handrails going to the front pilot near the steps so I formed my own from wire. The wheels are MTH scale wheels.
There wasn't much I could do about the noticeable gap between the trucks and pilots. This is a by-product of old Weaver tooling that MTH really hasn't changed in 25 years. If I was 100% committed I would have cut the frame in half and pushed the ends about 1/4" to close the gap. There is about a 1/4" gap between the frame and shell so a 1/2" filler strip would be required to hold the 2 halves together. New holes would have to be drilled for the smoke unit and shell but a little ambition and patience could see this through.