Mike,
The bottom 6" of the 48 is suppose to flair out to meet the fenders. The 46-48 Mopars are nor running board cars. Mine had the passenger side already cut off by a previous owner, the drivers side was still present, but it was so thin you could bend the metal with your fingers (of course, there were no rockers or floor on either side). I cut the driver side flair off. Rather then go through the effort of reshaping the body flair, I just ran it straight down when I built the new rockers, like the 41 body is. If you look close, you can see the kicker bars under the doors are attached to a flat piece of sheet metal where both the front and rear fenders still have the flair out that was suppose to match the body.

Other then the lower body flair, the two cars are pretty close to being the same. I believe 40 was the first of the "new" design. Then the 41s were slightly revised. 42 was another revision where I think they added the body flair. WWII stopped auto production in Dec 41, and civilian production didn't resume until late 45, which would have been the 46 model year. Very little changes were made between the 42 and the 46-48 model years. 48 was suppose to be a new model, but some issue came up and it was delayed until about mid year, Mopar called the new cars mid 48 or early 49 model year cars. Gene