That's why I kitbashed an SDP40 years ago, rather than an SDP45, which I would have preferred. I had the Kato SD45 chassis and handrails, and a Kato SD40 shell, so, with some styrene, I made a "quick and dirty" SDP40. It's in BN, no steam generator, so is a freight unit, but everything fit perfectly.
Why did I have a spare shell? The chassis is under a brass F45, which I simply could NOT get to stay on the track. Put a Kato chassis in it and it runs perfectly. This was long before Athearn released theirs, so I have three BN and one GN brass F45s, all on Kato SD40 chassis.