The biggest difference is the the '45 has a larger and more powerful engine. I think in rough terms the WD was about 38-40 HP and the WD-45 was between 45 and 50 HP.
Pretty much the same Engine but Allis increased the displacement in the '45 with either a larger bore or a longer stroke, and maybe both. My memory fails me on the details.
Spent a lot of time on both tractors as a youngster and you will defeinitely notice the difference between the two if you're doing heavy work.