Gary -
I disagree. I put a 420 pan on my 318, in order to install the 2 speed locking rear end. The ONLY difference between the two pans is that the 420/430 one has the extra holes punched in it. It is most definitely NOT any wider anywhere! Or longer, or anything. All the mounting holes lined up, and no issues at all (except that the powder coat paint on the 420/430 pan is flaking and needs to be repainted).
I think the extra width of the 400 series tractors is all at the front.