Found a Shapeways SDP40 with correct rear steps !!! Any body try these ??? What power chassis does it fit ??? Sent designer the same question, but I'm impatient. Could be third installment of BN Bicentennial trilogy !!!
If you have mechanisms for SD40/45, try those and let the class know... Alternatively, you could measure with calipers the inside dimensions of the shell (interior width, length and height), and the rest of us can measure our mechs to give you ideas. The designer of the shell may or may not have designed the SDP40 to fit the Kato SD40/45 mech as the truck centers are correct, but the wildcard issue is whether Shapeways would print it as-drawn or had the designer thicken the walls of the shell for printing reliability reasons. According to the Contemporary Diesel Spotters Guide (by Marre), the SDP40 was (by 1-1/2") SHORTER than the SD40. SD40 was 65' 9-1/2", SDP40 was 65' 8" long. Truck centers are probably the most critical dimension. Both SD40 and SD45 have the same 40' 0" truck centers as the SDP40. The pre-1968 production of the SD40/45 also had a 65' 8" frame, but it was lengthened by 1-1/2" after 1968, and no truck center changes.
Unless you have micrometer eyes, you'll never likely notice the difference. I'm thinking a small fraction of a millimeter.
Just multiple 1.5 inches by the scale (1/160) so you get 1.5 X 1/160 = 1.5/160 = .009375 inches. And if you can see a difference of 9/1000ths of an inch the math should be no problem.