35-40 vs 41-48 ford frame

Hi all, I was wondering what the difference between the above frames is, and if there is any advantage of one over the other. I believe that 1942-up trucks have parallel springs in the front. I have (i think) a '47 big truck cab (gas tank under the seat). Or if there is already a thread on this, please point me in the right direction.
Thanks, Mike

 

http://www.vanpeltsales.com/FH_web/flathead_drawings_ch***is-frame.htm

 

Wow, that's a cool site! Thanks!