John,
I can't say for sure exactly on your 40, but I put a set of big truck fenders on my 41 that i used to own. In the case of my 41, centering the wheels would have been more a little more difficult on a frame swap due to the large wheel openings. The space needed for the bigger tires made the litte 14 inch rims I was running look terrible. When I changed back to 1/2 fenders, the 14 inch rims looked more normal with a tire with a taller stance...the big truck fenders were just did not fit the look or the build. My 41 was more of a street rod than the truck I am doing now. The clearance with the ground did not look right either..1/2 fenders made it look a little like it was more in the dirt up front.
I guess what I am trying to say is..if you are doing a frame swap, I would get the correct fender first to make sure you get the look you want. It would be hard to go back and move everything on a frame swap if the look is not right due to the large fender openings.
Even if you found a steel fender in dire need of repair to use as a mock up, you might be money and time ahead...
my $02...
