After seeing the real thing in person, I am heavily considering re-building my #300 MOC.
When I first built her, it was a secondary build to my TSRR #500. I built the two locomotives so that they were relative to each other in scale, and I considered the BBB drivers to be in scale with the #500 drivers. Knowing this I build the #300 with drivers too large for it's size.
What I didn't realize was that the #500 drivers were a massive 74" in diameter, something I have since considered to be larger than BBB large drivers. In fact, the 56" drivers on the #300 is the size I've come to consider to be appropriately represented by large BBB drivers.
My idea would be to create a 5-wide cheese-slope boiler for the #300, making her either 7 or 8 wide, and probably much longer. This will also allow me to make the matching passenger cars 8 wide so that it will have two rows of seats.