RE: Centennial locos
I was at work when I answered the first time, so I could not get to my reference books. Sorry, but they do not have scale drawings, or I would try to scan them for you.
The Centennials were class 6900 on the Union Pacific. They did have four axle rigid trucks, but as mentioned above - you may be able to combine two, two axle trucks inside a fabricated side frame to simulate the four axle truck. Extending the width of this dummy side frame would allow some "wiggle room" for the two, two axle trucks. Perhaps only two or three millimeters on either side of the side frame would allow the trucks to flex a bit without spoiling the overall apearance of the model.
Modern Locomotives and The Second Diesel Spotters Guide both note that these locomotives had 6600 combined horsepower from their two prime movers. The locomotives were 98 feet 5 inches over the anti-climbers. The couplers may have brought the total length to 99 feet, as they extended out from under the anti-climbers a few inches. The control systems used on these locomotives were a test bed for the controls used for the Dash 2 series by EMD. One book stated they weighed in at 247.5 tons (US). While they were certainly magnificant machines, their long wheelbase proved their undoing - it severely limited their route availability. UP finally decided that coupling standard models together to get the required horsepower was more econimically sound than having super locomotives with limited route availability.
Best wishes for your kitbash,