Well, I am painting the car's underside now. I'm using MicroLux paints Roof Brown for the underside and will highlight some details with Rust. The purpose of my building this car was to physically see a comparison between HO and Sn3. I knew from simply drawing rails in 3 foot gauge, S scale, that Sn3's gauge would be smaller than HO. This stock car that I have built is actually a bit shorter than a standard 40 foot stock car in HO Scale. The car's height is very similar to the HO stock car. The only dimension that is bigger in Sn3 is the width of the car, which is approximately 1/4 again larger in size in comparison to HO.
So, what does all this mean to me? It means that as I had sort of assumed when I started, that Sn3 is about the same size as HO. In fact, I was very surprised that Sn3 and HO track is very similar in size, HO = .6496 in inches and Sn3 = .5625. The difference is only .0871, inside rail head to rail head, HO being bigger.