The Frankenstein shell fits happily on the Walthers mechanism with a single tiny modification to the sill. I've ordered a GP9 fuel tank from Atlas, fingers crossed I can make it fit. The mechanism has the cutouts for the air tanks, it's just a matter of how different the inside of the Atlas tank piece is from the LL/Walthers version. Incidentally, this third-version GP20 frame is hands down the single best running N scale mechanism I have ever encountered. The first-generation GP20s could already creep at <1 scale MPH; this one does that too, AND at top GP20 speed it still does not make any noise besides the clickety-clack of the wheels. It runs in absolute dead silence until the throttle reaches Bullet Train speeds.
Turns out waiting to work on the pilots was the right move. Out of the box the Walthers GP20 shell rides at the same slightly-too-high height as its predecessors (and as Atlas's GP9). But the combination of old shell and new mechanism rides magnificently, realistically low, and I'll have to shim the Z couplers accordingly: Between the ride height and the more accurate (i.e. narrower) hood width of the LL Geeps it almost looks too small compared to its oversized friends.
