Ford 1 1/2 ton trucks 1935 and 1937 frames question

[Ivan Podgornov]

Hi,
Can ou assist with questions regarding Ford 1 1/2 ton frames, please ?
Frames for 1935/36 and 1937 models look similar, however i don't have good reference details about.
It seems they have 38 inch width in the middle and rear parts and a bit narrowed at front edge (pic attached).
Are they compatible / identical ?

Then, starting from 1938 model, frame has 34 inch width across all the length.
What was the reason to make frame narrower than previous model year ?

Is there any reference about frame dimensions for 1935 and 1938, 1940+ trucks ?
I have some brochures with dimensions, but resolution is quite bad to read digits, except one on 1937 truck